It sounds like you hung in with him and his addiction(s) and ended up in true physical pain/exhaustion. Stepping away and allowing yourself time to heal, examine your own life sounds like it was the right choice to bring you back to a healthy place.
I have regrets over separating with my husband so quickly after his addiction was identified. He made it clear he didnt see a problem and didn't want to stop, but having no background we were both so naive about drugs and addiction. We did get back together; a little over a year later is when he basically came home and asked for help.
What is the status with your ex now? Did he stop using, or is he getting any kind of help? Have you been in contact, and how is it between you?
Questions I would ask myself; was the relationship healthy and fulfilling before the drug use began? Would you be willing to go through another relapse down the road ? How have you been doing on your own? Dig deep to find why you regret ending the relationship - i.e. maybe you are lonely, or the opposite feel free and happy and have guilt over moving on with your life?
If he is in the same place as he was when you left, then getting back into the situation would probably not lead to a fulfilling relationship. You might be better suited to cope now, and carry on with your own life in a productive way; but don't you deserve the whole package in a relationship?
Having said that, at the point my husband chose treatment; rehab, working with a therapist- I've stood by him, welcomed him home, and we both continue to work on ourselves, our marriage, and family.
I wish you peace in whatever choices you make going forward. None of the choices we make are easy, and most are made with many unknown variables in play. We just do the best we can.