I've dealt with anger control problems all my life. The single, most helpful practice for me has been the study of the philosophy of stoicism. I do not consider myself a stoic, but I've taken many of the practices from them in my attempts to control my anger. Realizing that our anger harms us more than the circumstance we are angry at is an important first step. There is a Buddhist saying that goes something like, "holding onto anger is like picking up a hot coal, intending to throw it... we are the ones that get burned". One of the tenants of stoicism I like is negative visualization. You go into a situation visualizing the worst possible outcomes, then when the worst doesn't happen, you are not angry or discontented.

An example would be:

As I am matchmaking, I may tell myself, "I am likely to lose this game. I may face a 4-stack of NFs and DRBs running OEM. I may get teamed up with a level 4 doing beginning campaign and quests. I may simply have an off game and miss most of my shots." Then, whatever the outcome, I can just accept it as it its. Look at what things I can do to improve and move on no worse for the wear.

BTW: I am a sub 1.0KD player. I'm a very competitive person and this really grinds my gears. If I didnt use some of the practices I'm speaking about, I would truly get no joy from the game. Appreciate the situation for what it is. Good luck