If you love this person, (which I assumed you did since you're sleeping with them, though everyone and circumstance is different and I'm not judging by any means)then tell them their negativity is affecting you. Ideally, they'll want to help you by improving themselves. Be gentle about it, though. No relationship is perfect, and if this is the worst part of yours then you're doing better than most. Just let them know, again GENTLY, that their negativity is affecting you. Let them know how to help you, without telling them to change who they are. A weird/hard thing as an empath. If they make an effort, that's very promising.