The best way is to not play it.
Not sure if serious.
If serious: Shrug it off and log out. That's what I do whenever I get peeved with something in any game.
Dog sitting. Can't get angry without the dog thinking I'm angry at him so I don't get angry so as to not scare the dog.
Understand it's a game....
Dragonflight Summary, "Because friendship is magic"
Don't play it cus its shit in its current state.
Do sports to relax and take a break if you are so absorbed that the game CAN make you angry.
As for the rolls, stop being an entitled little bitch. It's not YOUR item until it's in your bags.
When it stops being fun, Log off.
Simple.
Pet Battles or Fishing.
I find both quite theraputic after ingame frustration for some reason.
Especially when your minions somehow fail a 97% success on a Mission, while the team with 52% completes it just fine.
Every time....
Totally not bitter about loosing out on that Black Rock loot chest.
Nope, not me.
If it's over gear, just remember that it'll all be replaced in about 3-6 months anyways. Try and realize that this game is nothing more than a big hamster wheel (get gear to beat some thing to get gear to beat something else to get gear to beat something else, etc...etc...etc...). The grind (rep, gear, leveling, achievements) is only in the game to keep people playing/paying.
If you are having fun then go ahead and play. If it's that frustrating take a break.
I really only get mad when there is a great turtle in AV, and I can't get through the few alliance between the GY and there. doesn't happen often, but it has. Or, if it seems like we have a really good team, but the arena master healer and the 6 bloodthirsty guys are the bots, and the one getting the 100 kill achievement keeps asking if we killed Balinda or not and why we can't take Vanndar with 3 towers up, cuz he can offheal.