Firstly you need to realise that a negative mentality has probably cost your team a number of losses. It only takes one person with a negative attitude to bring the whole team down, even slightly, which could cause a couple of missed shots or a bad decision, cause a round loss which loses you a game.
Secondly you need to realise that while you play this game competitively, you also play for fun. Stop taking things so seriously! People make mistakes and while it's frustrating for you and your team members, you need to let people make mistakes.
I used to play on a team with a guy called Boomting (otherwise known as Josh / Joske). He was one of the biggest ragers / trolls in the European scene. Playing with him was a nightmare but he was such a skilled demoman that we had no choice but to continue with him in our team. After years of putting up with shit we decided we had to change his mentality.
I asked him to try something. When he got killed, I wanted him to credit his opponent rather than blame himself or his team. He gave it a go. I remember the difference this made. He would get killed and instead of, "WHAT THE FUCK? SOMEONE FUCKING PROTECT ME?!?", it turned into, "Omg, this guy just got an absolutely amazing meatshot on me... that was amazing!".
It kind of put a positive spin on moments that would have been shit for us. Hearing our team mate struggle puts us in a bad mind state, but crediting the opponents instead of blaming himself / us didn't effect us in a negative way. If anything, it added a few 'lulz' to our mumble during games. Our matches became less stressful, and we started to perform a lot stronger.
Now it sounds like you're also kind of causing arguments or rage in comms, so I'd suggest trying that methods yourself. Even if your team are raging, just laugh about it in your head, and never say a bad thing on your comms, only credit people. If a scout jumps in, kills your medic around 4 of your team, then it was the scout who was amazing... not your team who were shit. Adopt that mentality and the game can become a LOT more enjoyable. It's all about being the bigger man. Sometimes you will want to shit talk, but you just have to hold it back and try to keep your team's positive mentality.
Honestly, people underestimate mentality in gaming, but notice how a sniper who misses 6 shots in a row will also miss the 7th, but the sniper that just hit 3 headshots in a row will hit the next 5 and destroy the whole team. It's simply because they KNOW they are going to hit the next shot, whereas the previous guy is trying too hard to hit because they missed so much.