Dear Donini...

Help. Climbing with my buddy is a downer

So one of my good buddies is also my main climbing partner and climbing with him has been a freaking downer lately.

He gets really upset whenever he can't send a route or problem after one or two tries and it makes him look like a pouty little girl. He punches the wall and walks around like he stepped in dog sh#t without shoes. The thing is that these routes/problems are above his limit yet he expects to magically get better without putting in the hard work and effort. I've tried bringing it up to him and nothing changes.

I tell him that climbing should be fun at the very least and that he shouldn't be climbing if he is going to leave the gym/crag all fumed. I tell him that if he is pushing his limit that he should not only expect to fall, but to fall over and over again. He tells me how much better I am than him at climbing but in a way where he is putting himself down but it is because I worked my ass off (I sent almost every V0, V1, and V2 before even touching a V3 at the gym). I could go on and on.

The worst part is that when this happens his safety goes out the damn door. We were climbing on TR the other day and there was literally 15 feet of slack in the system which I know isn't a "big deal" compared to lead falls but it's not necessary. (I was climbing on an overhanging route and the rope was drooped over my shoulder and I couldn't see it, otherwise I would have stopped sooner) He stops doing partner checks and I know that it is stupid/mindless stuff like this that is going to get one of us hurt/killed in the future (if I keep climbing with him)

I already have multiple climbing partners but I want to keep climbing with him based on our pasts. How would you guys approach this situation?

TLDR
My good buddy is my main climbing partner. He becomes a whiny little bitch whenever he can't send and his safety goes out the window. What would you do?