First of all, I want to say that I think it's really awesome that you've become aware of this. It's the best place to start and I want to commend you for that <3.
A few things that helped me a lot:
Try to have some banter with your team in champion select, especially for ranked games. The more you like your team the more understanding you'll be towards their performances.
When your teammate starts feeding just remember that we all feed from time to time, and it's not the end of the world. Assume that your team feels really guilty and helpless. They've played poorly, probably lost the game and feel shitty about it. It's much easier to have compassion for them this way. When they blame you, that's the helplessness showing. Sometimes all they need is some patience and the opportunity to help make up for the feeding.
Take some time out of the game after every match, and if you feel salty, just take a longer break, do something you love and try not to dwell on your losses for too long. If something really drives you up the wall, then find a forum post complaining about that thing, just knowing that other people understand your frustration can be a big help in calming you down.
Another great idea is to have a game mode that you don't take so seriously, and play that game mode when you want to just take it easy.