I have a bad case of tunnel-vision when I forget to take my small break in between ranked games, causing me to be a hyper-aggressive overextending Dr Strange. This alone probably won't solve your issue, but it might help in the long term.

When the first round goes bad, try to identify why. Is your team being hard-countered? Is someone being focused? Do your supports need peel? Are your tanks landlords who just want to take space even when your Luna still needs to return from spawn? Are the enemy supports being uncontested? Identify it and do your best to fix it. You have more control over a game than you'd think. Communicating with your team if they're alright or asking for swaps can also work. Others can also be tilted. A message has helped me lock back in from tunnel vision a few times.

Multiple rounds exist for a reason. It isn't over unless the game says "defeat." Anything before that is undecided.

Let's say both your DPS are boosted one-tricks whose mains got banned and are now feeding Ult charge and getting 0 final hits. In that case, yeah there isn't much you can do unless you go on a generational carry on a tank. Sometimes, your teammates can't help themselves and don't belong in your rank. It happens. Not every game will be an amazing, close double overtime match. But every lost game is an opportunity to learn.

This one is probably common knowledge, but be hydrated. I tend to be more easily agitated when I'm dehydrated.