aww, honey, i'm sorry you're having such a rough time with this. it is a crappy situation, i know. right before i kicked my ex out this past summer, i had a couple opportunities to play around with some of my male friends, no strings attached. as much as i really wanted, to, and as much as i knew i was going to leave him soon, i just couldn't do it while we were still living together. one of these guys is a total sweetheart and great friend, and we've had some teasing/flirting going on for quite awhile....but i felt like it would be unfair to me, my ex, and my friend to get into any kind of relationship (or just being fuck buddies) while i was still living with my now-ex.
the divorce is still dragging on, (cuz we can't find the bastard to get the papers served) but we've not lived together since early august. i met someone in september and we started dating in late november. we're both very much in love, and he's moving out here to be with me in july (providing the plan to get the ex served with papers this week is successful). right now...despite what my mother and brother think....i don't feel like i'm cheating on my ex. there's no emotional connection, and hasn't been for a long time. it's been over a year since he and i had sex. we haven't lived together in 9 months. and i'm not still married because i'm not actively trying to get the divorce. it's just rough when the other party won't cooperate. but there is no chance we'll get back together and i have no feelings for my ex, other than the occasional burning desire to run him over with my truck.
so, from someone sort of on the other side (still technically married, but involved with a new partner), i guess i'm wondering why he let himself get involved with someone before he actually took steps to end his current relationship. it's not like high school, where you can just quit calling the person and hope they get the hint that you don't want to see them anymore. even when i knew my marriage was over, but before i finally punted him out...i couldn't bring myself to even have casual sex with somebody else. and i certainly wouldn't have been in a relationship with someone and promising them they were next on the list. honey, do you really want to be the second choice on his list? because if he's not standing up to his wife and making time for you two to be together.... despite what he tells you, he IS making you his second choice. and that's not fair to you, both children involved or his current wife.
i know it's hard to not love someone. and i know it's hard to leave someone, even if you think deep down it's the right thing to do. so i think a good compromise for you is the "drop dead date". talk to him and make him choose. he has 60 (or 90 or 180) days to make a decision....either continue to live with his wife, and you will move on..or stand up to her, leave her, and choose to be with you. but he HAS to choose. and this way...you can feel like it's his decision...and not feel like you made a mistake walking away from what could have been a great relationship. it puts the ball totally in his court and makes him responsible for his actions (or inactions)
good luck. things *will* work out for the best, hon.