My take on this may be a bit different from others'. I don't think it's ever a good idea to start seeing someone else if you're in a primary relationship that has serious issues or is not stable. I don't mean perfect, but I mean one that has open communication between partners. It sounds like yours was troubled on a number of levels. If you really wanted to work things out, you would need to work on that first, and then open it up.
From what you've written, it sounds like you were not happy in your marriage, and he might actually not have been either, but you weren't really communicating or fixing it. Your new relationship has given you a way out, a way to bring things to crisis. That's not a judgement, I think most people end up resolving things in these sideways fashions. Your needs are now different and you need to acknowledge it. Going forward though, if you have another primary relationship, I'd caution you to really think about my first point.
"Love takes off masks that we fear we cannot live without and know we cannot live within."James Baldwin