Sex post divorce

My wife and I separated earlier in the year. Recently, I had sex for the first time in 20 years with someone who wasn't her. The sex was ok. It was a little different than what I had been used to. But afterward, I had this overwhelming feeling of regret and sorrow. Not just for myself or my ex, but also the woman I slept with. She has been persistently contacting me since. Wanting to get back together again. And I just don't want to do it, but I also don't want to make her feel like I had used her. She's a wonderful woman, but I feel like I can't do it again with her as I feel terrible about the situation. And now I'm afraid I'll have the same feelings of regret with different partners in the future. I'm enjoying dating again. Meeting new people and having a great time. I enjoy intimate moments, kissing, cuddling, etc, but I'm afraid that I'm going to have regrets if I sleep with someone else again. I would love to have these relationships blossom in that direction, but don't want to feel so hurt and confused afterward. What can one do in this situation to move passed these feelings of regret?