Hullo and welcome!
Few immediate thoughts;
1) You say you experience sex with him as painful. Is that a physical or emotional problem? Do the two of you (or you in general) have trust issues? What made you feel your first experience outside of this relationship was horrid?
2) Have the two of you had sexual therapy? Is it clear that you don't have orientation issues, in that either one or both would be gay or asexual? You have a disconnect of communication going on which could improve with therapy, methinks.
3) Have you had sexually satisfying experiences after the two of you decided it was okay to seek out partners outside? Yet again I'm wondering if you might be asexual, have incompatible kinks, trauma or if you just need to connect on a more profound level with someone in order to experience sexual joy.
4) Don't ask don't tell, like anything that involves closeting or lying by emission, is hugely stressful, and you want to move on. What is your ultimate goal? Reconnect with him? Be able to live free from fear of discovery and enjoy your other loves openly? Build a poly family/intimate network together?
You might benefit from learning some basic non-violent communication skills before embarking on this project. If he is not willing to do couples' counseling, consider getting into sexual therapy on your own.
Me: bi female in my twenties