Really? Blimey, I thought I might be chancing my arm there, but it just sounded so familiar bearing in mind a few friends' fucked-up relationships with their dads.
Look at this realistically though. This bloke you were involved with was selfish, that's all. Yes he might have said he loved you, but expecting a woman to play second fiddle really isn't love. Any more than he loved his wife/partner enough while he was seeing you. If he's got a chance to make that up to her now, I hope he takes it. And if he still doesn't love her enough I hope she sees it and boots him out.
But don't go thinking that you messed things up. For a lot of people having affairs, it's all about them and not the actions of the OW or their partners. So they choose what's best for them personally. Being scrupulously fair about this, sometimes having an affair makes someone realise how much they've got to lose especially if they get found out. And the OW/OM was just the bit-part player in that realisation. That's hard for that person to stomach, but although you were led to believe he was your best friend the truth is it probably isn't personal to you or anything you did or didn't do. You were there to fulful a role at a point in his life and his other relationship.
What would be more productive is to find out why you agreed to play this role, rather than dwelling on behaviours that you think made him choose his partner over you. With the friends I've mentioned, the hardest thing they've had to confront about themselves is their own selfishness. A lot of OW use the get-out-of-jail card of low self esteem and although that might be a part of it, it's never the entire reason. In friends' cases it was all sorts of things: selfishness, competiveness, low esteem about things like looks/weight, an over reliance on men and romance to complete them, a degree of gullibility with men they fancied, not enjoying their own company and developing self-reliance and in a few cases, difficult relationships with the first men in their lives: their fathers. It's only fair to say that a few non-friend OW I've met just didn't much like other women, but that doesn't really sound like you.
If you baulk at the word selfishness, think about what it would have said about you if he'd chosen you? Sure, you might have excused yourself because you were in love, but building a relationship on the back of someone else's pain is selfish isn't it?
Can you afford some therapy? With someone who'll challenge you and won't let you make excuses? Or do you have a really good woman friend who will be critical and honest while still loving you?