Hey there,
Is you BF getting some sort of professional help? He really needs to be seeing someone, because it's not fair for this to be all on you. Those sort of conversations are incredibly draining and it's not surprising it has this effect.
If he is seeing someone, as hard as it is to do you may need to try and wean him off you, to an extent. Encourage him to talk to them, and try and get some coping strategies in place. He needs to learn to I guess self soothe in a way, because as good as it is to rely on people this isn't a sustainable behaviour.
If he isn't seeing someone, he really should be and you should encourage this. If he refuses, well I think then you may need to do a 'cruel to be kind' sort of thing and straight up tell him that he needs to see someone and take responsibility for his illness, and that when he does have these long intense conversations with you it triggers you. Reassure him that you do still love him, and you do still want him to tell you things, but he needs to understand that him constantly refuting your care and telling you you're lying/can find someone better etc has a really negative effect on you.
Aside from that - in the meantime after these conversations, just take some time out. You've been so strong so far in staving of these
SH urges and combatting this addiction, so keep using whatever coping mechanisms that have been working. You can keep this at bay - you have since Jan and you can now.