I can understand your jealousy. I felt the same way during my divorce proceedings (which went for 6 months) and I did some occassional, what my girlfriend and I called "drivebys," to see if my husband was at his apartment or not. On those nights he wasn't there, and I didn't know where he was, I felt very uneasy.
I remember the jealousy being difficult to handle because I was working on rebuilding a friendship with him (being a great listener about money, work problems, etc... and just supporting him. For quite awhile this friendship was one-sided. I was the "giving" one!). Also, we had a pretty steamy intimate relationship going on as well and that can be a real tightrope to walk. Detachment can be very difficult for women in sexual relationships (somehow you have to ignore the effects of oxytocin). It took time, but eventually I did learn to emotionally detach while still having an intimate relationship with him.
Anyhow, the one thing that helped me during this time was to keep forefront in my mind that my husband needed to be out there dating, dealing with OW, etc... to realize where he wanted to be. I didn't want him back unless he truly wanted to be back. I wanted him to experience OW, dating, etc... to get it out of his system. Otherwise I figured he might come back and we'd just go through the whole giration again. But, as a matter of fact, we had almost gone through it in the past! He had tried leaving a few times before and I always convinced him to come back. So this time I completely "let go".... at least to a certain extent..... I told him I'd be his friend and I'd be here for him if he wanted me, but if he wanted something else that was fine with me. I wanted him to "go out and find his happiness." At times it didn't feel good knowing he was involved with OW, and as that died out, going out to dinner with women his sister and others set him up with.
But this was something that needed to happen.
Also, I needed to find my independence. Even through I was "there for him," and maintained I wasn't in any serious relationships (and wouldn't be until the divorce was final), I did go out a lot with a group of girlfriends I found who were also going through separation and divorce. I made sure my husband saw me looking absolutely amazing, and I'd be mysterious and somewhat evasive about where I was going. Eventually he knew I was going to nightclubs and that I had male friends. Also, he learned that some were interested in dating me once the divorce was final. It was kind of ironic, but very slowly the tables turned. I started to lose my jealousy and my husband started becoming increasingly more jealous.
Oh well.... that's my story. I can't say it would be the same for you. I wouldn't give an ultimatum unless I was definitely going to follow through with it. I personally think you're better off working on emotional detachment and focusing hugely on your own life (with the idea a divorce "will" happen and having a great life in place so if it does it won't be such a painful blow). The best piece of advice I think I got during this whole thing was to give my husband "a lot of leash."
There is no arriving, ever. It is all a continual becoming.
