Stop looking her up or (what would you insert here?).

Are you willing to follow through with whatever consequence you throw down? If not, then she knows you are not serious.

Perhaps you could reframe it. Instead of an ultimatum, call it a boundary. In trauma groups, people talk about boundaries all the time. Boundaries are something you put in place to make you feel safe (emotionally as well as physically) while you work on yourselves and your marriage. So, you would tell her how it makes you feel, and then say if it continues, you will ______________. You are doing it not to punish her, but to protect you.

I think Stronger makes an excellent suggestion here. I know I didn't react well to STBX's ultimatum that he wouldn't enter couples therapy with me and tell me what he had really been doing all these years unless I promised I'd never leave him. That pretty much put the nails in the coffin for me. Recasting it as a boundary is better, but consider Laurence's cautions--and my own experience--no amount of pressure (or loving support) stopped STBX from going right back online again and again and again to arrange trysts, sex chat, look at porn, etc. The crux is that they literally are not thinking of anyone but themselves. I am not even a three-dimensional person with feelings to my STBX -- I often wonder if he actually has damaged or undeveloped Theory of Mind. It's quite possible. Anyway, good luck. My dad, a guidance counselor, would say to me when I was young: you can't MAKE anyone do anything. If it isn't their choice to do it, if they can't imagine why you would want what you want and then freely offer it, then you don't have much to work with in terms of shared values and caring.

Lilly, we are trying to live amicably right now as there are multiple family events and vacations already planned and paid for. I just want peace and it is not looking like I'm going to get it as long as we are still together.

When I first confronted my husband, I really wasn't sure what I was going to end up doing, but I did have one ultimatum.  I said I refused to be lied to in my own house.

It didn't actually change his behavior, but there was one thing it did for me: it committed me.  It made me stop and think what my own personal "line in the sand" would be.  When it finally happened, there was no room for me to back out or second-guess myself.  There was no "am I overreacting" moment.  I hadn't promised my husband that the marriage would survive, but I did tell him that I would not be lied to in my own house, and that was an absolute dealbreaker.

So when it happened, I knew that the marriage was only important to one of us, and it was time to throw in the towel.  He was devastated, and to this day he tells me he doesn't remember me giving him any kind of ultimatum, but he heard me loud and clear.