I've had those moments. It happens usually when I'm trying to unwind from something else (job, family, etc) and the losing in Splatoon just makes life worse. So, first, assess what else is bothering you (if anything), then remind yourself that sometimes Splatoon isn't the best way to let off steam.
When I notice I'm getting more frustrated than I should, I try a weapon I KNOW I'm going to do poorly with. That way, if I win, YAY. If I lose, meh, it was the weapon. After you go on a serious losing streak where you K/D and points are low, then you will often get paired with less competent players and can do better. Try experimenting with builds and weapons to shake things up.
Also, assess how you are playing. I often THINK that I just want to play support and sit back and ink, so I pick a long range weapon like a squelcher. But then I start pushing forward, either because I need to, or I just feel inclined to. This often goes badly when I try to play frontline with my support weapon. But if I switch to a weapon like the .52 or tentatek I do much better. This is because, for some reason, my playstyle that day is more aggressive and I need to play a weapon that helps me do that. This works in reverse as well.
Good luck!