hi Maggie17, and

i came to this site after my relationship had ended. i still had hopes for reconciliation. i had a lot of the same questions.
theyre good, important questions. these are relationships that you should go into with eyes wide open. know the pros and cons. know what success looks like to you personally, and what the likelihood of it is based on you both as individuals, and on your relationship.
for starters, substance abuse adds a really difficult dynamic to an already challenging relationship. people with bpd traits do not have adult life/relationship skills. it may be one of the most significant challenges of your relationship. for example, if he were in therapy, it would be the primary focus - it exacerbates any maladaptive personality traits. overcoming that will be a tall order. the tools will help you. they will not lead him, necessarily, to recovering from it.
When my boyfriend reaches out that he is ready to talk, I intended on letting him know that he once mumbled that he has BPD.
i would strongly suggest against bringing this up when he reaches out, or any time soon. bpd is a shame based disorder; it has a lot of stigma surrounding it. it means a lot that he mentioned it to you; it means that there was a level of trust. in the current circumstances, trust is diminished.
there is a good chance that he would connect it to the fact that you told him you were questioning the relationship. while it may feel like what would be a well intentioned statement of support, im not sure thats how it would be received, right now.
hang onto what you know. the fact that youve found a support group (i know of no better place), and are searching for a therapist for yourself, are two of the best possible things you can do for yourself. in terms of shifting your relationship into a healthier trajectory, play the long game. learn what youre up against. honestly assess if youre up for it. learn the tools here. work out a plan with your therapist; bring it here for input from other members.
in the short term though, its a good strategy to give him space. you dont want to over pursue or chase if hes retreating.
if you want to reconcile, giving him a smooth landing is likeliest to go over best. what does that mean? do a lot of listening (
https://bpdfamily.com/content/listen-with-empathy). let him blow off steam. get a sense for where hes at before you say your piece. when you know those things, youll be in a stronger, more confident position as far as going forward.