Sorry this happened.

Sometimes people are closeted about their mental illness diagnoses, not just a bipolar diagnosis. They're afraid of being judged in a negative light.

Perhaps his ex-wife broke up with him because she found out about the bipolar diagnosis and couldn't mentally handle it. I'm not saying that's the case, but it could explain why he never told you about the diagnosis. That, or maybe he was judged by other people and doesn't want to be hurt again. There are a lot of possibilities about why he didn't tell you, basically. So the problem is not necessarily YOU, but other people instead.

You can definitely stay in contact with him like you're doing right now, but that's up to you. But remember, not every behavior is due to bipolar and not every behavior is an excuse. For example, cheating? Not acceptable. (I'm not saying he's a cheater, but that's just one example of something that is inexcusable.) It can be hard to differentiate between bipolar behaviors and personality behaviors, but it can be done.

Whatever you do, please don't tell him that you've discovered his diagnosis. He will tell you when he's ready. Otherwise, it feels like you're talking behind his back and it can hurt him. (I know it would hurt me.)

If you can't handle the bipolar, that's okay. Not everyone is cut out to be a "caregiver," or so to speak. It can be hard on both people and may create tension that you can't handle.

You can always offer your support (like you've been doing) and tell him that he is free to be open with you. Assure him that you won't be judgmental and say you're always there for him.