Watching the replay usually helps a lot to get the salt level down.
This is the best advice you can give. I was once pretty salty after I lost a sniper and I was absolutely 100% no doubtly sure that he used a map hack to chase my unit. But in the replay I saw that the sniper was visible for a split second. I flamed my enemy ingame and I am still ashamed and sorry for this. Of course I lost the game.
Every time I lose it feels like I lost because of bullshit. [...] When I win it's pure skill, when I lose it's because the game is shit.
I know your feel. But still now (haven't played in some days and I am totally calm right now) I think that the Allies have way more options for cheesy strats (I play Axis 99% of the time). Sure, the Axis also have a lot of bullshit but that's mostly behind p2w commanders I don't own.
My 2v2 buddy is way worse. I swear a bit out loud sometimes, he full on quits the game for weeks at a time after a particularly bad loss. And frequently makes snide comments to the effect of "NICE BLOB" in the chat. Lol.
I hope he knows that winning against such enemies is the absolute best feeling you can have. He gives the enemy much more satisfaction than he would give them without flaming.