opinion

My first wife and are great friends. It helps that she's remarried and 800 miles away.

Your deal is more complicated. You have to deal with a current boyfriend, and I'm not sure I would care for a girlfriend spending more time with an ex than with me. My opinion is that if you want to keep the current bf, you're going to have to spend a great deal less time with the ex.

Just my opinion, which is all you asked for.

My apologies in advance as I wrote this early in the day so I can't vouch for how coherent it is.

IMHO, your stance on this can hinge on a few issues (such as whether the ex and you were married):

If there was a divorce:

The reasons for the break up. If the reasons were massive incompatibility or integrity issues like cheating, abuse, negligence, insensitivity, or just not being a good partner (for example, wild spending without regard to budget) - it might be hard to EVER be friends again.

If as a result of the divorce one person massively has to adjust downward their lifestyle..loses house, community standing, etc. all bets are off.

If there is children and custody battles ensue, involved IMHO, all bets are off. Often the best that can be expected is a civil arm's-length arrangement.

Sometimes when you're in your 20's, one of the partners is at different maturity level from the other. After the trauma and settling in after a divorce...one of the parties smartens up after the time away. Maybe after an interval, there can be a type of friendship.

However, when you're divorced ... say in your 40's, maturity might not be such an issue (hopefully) and things can be a bit easier for friendship again.

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If there was no divorce and it was a bf-gf relationship (but no kids or property):

It's mostly a 50-50 proposition. However, if one of the former partners is in a new relationship - you have to keep some sort of distance from old partner if new partner is at all complaining. Those dynamics will take awhile.

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FWIW, I stayed friends with both of my exes. However, I had no children and we mutually agreed on property settlement without the use of any lawyers.

Also, FWIW, I was over 40 after my divorce and/or breakups.