Blueberry, 
Reading what you say about your ex's forcing your son to watch him change into women's clothes makes me massively angry.  That your ex cannot spare a minute to consider your son's feelings about his father's transition is unconscionable.  

I know your son said he is "ok" with his father's transition, but even if that's true, it's going to be a massive readjustment of their relationship, and will call up feelings your son doesn't yet even understand he'll feel about his own sexuality and gender identity.   Would it be possible for you to take him aside, and to tell him that his father should not have put him in that position of watching him change?  It's really important for you to tell your son that although he may not realize it yet, his relationship with his father is going to undergo a change, and he needs someone he can talk to about this--someone who is not his father or you.  He needs a counselor, and not a "gender" one who will consider that the son's job is to get with the trans program.  Your son may not see the abuse, but it's going to affect him.

I have to say, your ex's actions so remind me of my own ex's actions.  My ex never hinted anything about a confusion about his gender or any dysphoria.  He discussed nothing with me, and hid the porn use and the "experimentation" he was doing  (talking informally to an ex student with whom my ex had always had a relationship that crossed prof/student boundaries, and who had become a therapist).  I knew nothing about any of it until one day he walked into the house and dropped his trans bomb, announcing he was "a woman in a man's body" and was planning to have surgery to have his testicles removed, to take female hormones, and to transition.   And then, as with you ex, it was full speed ahead, wanting me to accompany him on the ride and affirm him and celebrate him, and assist him.  

Stay strong, Blueberry.  None of this is your fault.  
 

Blueberry,

    I am the ex of a trans-identified man (aka a "transwoman").  After my ex disclosed, I stayed for three years, trying to wrap my head around him, trans-ness, and the thoughts and feelings his disclosure, words, feelings, and actions raised in me.  What you are saying about the way your ex accused you, and the way you are reacting, rings very familiar to me.  So this is what I from the perspective of 11 years past disclosure, and eight years past moving out and divorcing, want to tell you:

His porn addiction and his "feminine feelings" have nothing to do with you.  They do not have anything to do with your loss of libido from taking antidepressants or from your undergoing perimenopause. 

Nothing you did or failed to do "caused" him to do anything.  He made the choice to view porn and to fall down the porn-fueled rabbit hole. His choices and actions are not, and were never, your fault.  

It took me several years after disclosure and leaving--so at least five years--to realize that a pull to trans porn and trans content in general is not simply one choice among many, but that a man who chooses that path, of trans porn, cross dressing, and forced feminization has a predilection toward it.  It's not just one choice out of many out there.   

Straight men on this site have been known to tell women here who are struggling with their partner's "explanations" that straight men never choose to view gay porn, although a gay in denial or closeted spouse might tell his wife that his choice of gay porn and gay hookups are just "curiousity."  It's a specific choice our partners chose because that is the way they are.  

I repeat: the choices your ex made were not a result of your antidepressant use, your perimenopause, or anything else about you or what you did or did not do.  They were his and his alone.  You are not to blame for them and not responsible for them.  Full stop.
 

Shelby, You really hit the nail on the head.  What galls is that they were willing to sacrifice us to their needs.  Maybe they were unsure about their sexuality/gender identity, or hoping they could sort themselves out by marrying us (as apposed to downright knowing but duping us), but even so they used us, "strung [us] along while they did what they needed to do to figure themselves out."  Then, after disclosure or discovery, they wanted and in some cases demanded that we remain with them and/or supportive of them.  It's maddening.

Alone38,
   Ex-wife of a trans-identifying man here.  We'd been married 32 years at the time my now-ex walked in one day and announced he was "a woman in a man's body."  The behavior you describe is very familiar to me, as I experienced it with my now-ex, from the bald announcement that he *is* transgender, which probably means that he's been turning this over in his head for quite some time, on his own, not sharing it with you.  That he hasn't been willing to share any of his questioning with you until the moment he dropped his trans bomb is not a good sign for expecting open communication from him going forward.  Or, at least, it certainly wasn't with my now-ex. 

 I also heard the "I want to stay married" but "I'm doing this" line.  Together with his feeling that he'd been vicimized by being born male, that added up to his feeling entitled to having what he wanted, which was to live out his fantasy he was a woman and have me there to serve as guide to womanhood, to validate his new gender identity, and to acquiesce to and act in his newly redefined sex life.

That your husband has started to be secretive is a red flag.  It means, at the very least, that although he says he wants to remain married, he does not want to communicate with you about what he is feeling, which is not the behavior of someone who is committed to staying married.  His secretiveness might also mean he has an online community he's interacting with, and, fi so, that online community is likely egging him on.  His secretiveness may also indicate that he has already taken steps to, as the trans community puts it, "feminize" himself, but doesn't want to tell you.  For some trans-identifying men, my ex among them, the secrecy is part of the turn-on.  

 I understand your anguish.  I felt it, too.  It's very hard to be ambushed by an announcement of something you had no idea about, and that fundamentally alters your past, your present, and future.   You will need to have support,

Moira,
   On the off chance you don't realize that the "reply" feature doesn't work on "Our Stories," I'm opening a thread here with a comment for you.

   If you cannot divorce, perhaps you might explore whether there is a way to stay married but live apart.  This is what I for a time considered trying to do with my now-ex.  This would, of course, entail finding a way for the two of you to live separately., and would depend on your finances. I ended up divorcing my now-ex (who is still closeted), but when we first separated, and shared a bank account, it was cheaper to stay married and fund one apartment (where I lived) then to have two separate housing expenses (our house was paid off).   I realize you may not own your home outright (no mortgage), but even then, you might explore alternatives like going to stay with a family member, or house-sitting either locally or nation-wide.    

   Just putting that out there.  But let me add ths: for a long time I told myself that what was holding me back from leaving was financial.  Over time, however, other factors came to weigh more heavily, and the scale tipped so that even though my financial situation was not what I had wanted it to be, I felt I needed to be out, and I left anyway.  I was 64 at the time.  

   I'll also say this: my father was bipolar, and life with him was hell.  My mother endured for decades until his illness crossed the line and he threatened a family member, so she left.   Please know that your husband's mental illness is not your cross to bear, or yours to manage.  I would hazard a guess that most of the time your husband is able to get along in the world, which means he is also capable of seeking help and adhering to a medication regime.  If (like my father) he won't, he simply isn't capable of being a partner, and you have no obligation to stay to honor a partnership.