What do I do.... HELP!!

I have been in a relationship with my partner for 13 years this year, we have 2 kids. At the start of our relationship it was great, after we had kid no1 it started to change, we spent less time together, I felt I was the only one trying in our relationship. When kid no2 came along it got worse, we have spoken and nothing has changed, I'm the one to makes time for us, I'm the person who makes any effort in the bedroom and when that does happen I don't know if I am making her feel good. That being the case I sadly have started to look for what's missing elsewhere. For the last two years I have developed feelings for another woman to the stage that I love her. We are so compatible, the sex is amazing, we love the same things, we enjoy cooking for each other etc. Sadly she doesn't understand how hard it is the leave your kids behind and the emotional strain thinking that puts on you trying to make the decision. So 6 weeks ago I got the ultimatum "it's me or her". She asked if I wanted more kids which I replied not at the moment no which she took at no never and she said "well if you can't give me kids there can be no us" so I left thinking about if I could grow to want kids. 4 days later she is in bed with another guy. Since then we have spoken and she has said that she is seeing this guy and the dreaded words "but I don't know what I want, I need some space to think about what I want". I said that I would respect her wishes but said it will be hard to find out what she wants when she is seeing and sleeping with another man, which she says I am wrong thinking that. At this moment I am fighting to save this relationship and get her to see that I want her and I have offered her everything I can, that I know she wants. I know I need to give her this space but the urge of wanting to tell her and text her is overpowering. Last night during this thinking period she vanished in me and went to his ?. I don't know what to do I love her and want her, she says she really does love me, But I think I am being strung along until she says I'm in love with the new man. I just can't take anymore of watching her grow with him and forget me when I am fighting for her. Advice please.

What do you want?