This is a question I have as well. For now we have decided to separate when she moves out of the house. A friend at work is buying a house and has offered to rent her a room. She will move out in a couple of months. Financially it will make sense to separate. We have discussed that if in the future, we would have to divorce if one of us wanted to get married again. I like the separation idea because I am stupidly still thinking that we may find each other again in the future( still having a lot of trouble letting go).

I second Phoenix's advice.  Consult a lawyer and ask him/her this question.  When I asked the lawyer I consulted about separation vs. divorce she said she recommended divorce.  A separation adds costs in the long run because you pay the lawyer first for the work on the separation and then for the divorce, and, while you are separated you are still financially responsible for some debts your spouse might accrue (health, for example).   A lawyer will also educate you about what the law considers equitable.  Phoenix is right that when things get down to the brass tacks of money your partner who had seemed to be so reasonable might become less so.  My ex certainly followed that pattern.  One advantage of being the one to initiate divorce is that if you are the first to file, at least in my state, you also end up with an advantage in court should there be fights in front of the judge (you speak; your spouse speaks; you get to rebut).  
 Divorce is not a time to get sentimental. Divorce is all about custody and property.  You have to protect yourself financially, and, if you are going to have custody, you need to get the best deal you can for your kids.

Thanks for the advice all. My spouse has mentioned that she just wants to separate, and she can see herself coming back to me. But I think that is just her trying to keep me on the back burner if she crashes and burns. And I don't think anyone should relegate them to back-up status when they've provided, loved, taken care of, and committed themselves to someone for over 15 years. If I'm not good enough to be top priority, then it's over. Also, although I feel like I haven't fully processed the infidelity, I do not think it's likely that I will ever be able to trust her or feel safe with her in the long run. So even if she picked me as her number one, I don't think that is what is truly best for me.

From a logical stand point I'm pretty certain of the best way to proceed, but it's very hard to get over the emotional connections and hopes of what was supposed to be.

phoenix wrote:

My advice:

Before initiating divorce proceedings, please take some time to prepare yourself.  When divorce proceedings start, people change and become more selfish and sometimes it becomes a battle.  
Consult attorneys in secret to get advice and education.  Find all financial documents.  Make sure you have access to checking and savings and retirement account numbers and info.   

Most states require anywhere from 2-6 months of time for divorce proceedings.  You can change your mind at any time and cancel.   Might as well go forward with divorce so that if you chose to proceed you will not have wasted the time of separation period.