I (34H) met Anna (25F) at party. She is in a long term relationship with Jimmy (28H) for 5 years. They never shared "serious" projects (they don't live together, have never talked about children) but their families know each other very well. She rarely mention him but if i ask, she would say she love him without a doubt.

However, she said they were in a bad period and she cheated on him with a ONS a few months earlier. She told everything to Jimmy who was angry but forgave her. A bit later, she asked Jimmy to open their relationship and (my opinion here) "forced" him to do when he didn't want her to see other men.

We met with Anna at this time. She told me very early she was with Jimmy but we started to date 2/3 times a week, being closer and closer. We spent night together at bars, at our places, bathed together and ultimately have sex. She said Jimmy will never be informed because she's not cheating considering their newly open relationship. We are now spending time like a couple, entire week-end together and often have sex at both our places.

She told me Jimmy would be mad at her if he discovered our affair so I asked her to leave him and only date me. She refused because she doesn't want to leave him, because he was faithful, loyal, he loves her deeply and he didn't do anything to deserve her to leave. She doesn't want to leave her stepfamily that she know for a long time. On the other side, she said she loves me as well and dreams about us being in an official relationship. She now spend 2/3 days/night each week with me when she sees Jimmy only an evening/week. She told me she feel guilty but doesn't want to end her relationship with either me or him.

The problem is I'm growing more and more feelings for her and I can't understand her mindset. - Is it possible for a woman to completely love 2 guys that much ? - Is it possible for her to face Jimmy every time knowing she cheated on him multiple time the same week ? - Should I leave even if I'm convinced we're pretty made for each other and the only problem is Jimmy?

I'm pretty open minded but I can't understand that kind of mindset in an open relationship...

Thanks a lot for your help :)