I wouldn’t say over thinking’s a problem if by that you mean you try and play mind games with your opponent and just eat hp srk repeatedly, many people online just put no thought in to what their opponents doing at all, they might as well be playing 3s’s car smash stage throwing out any random move - what you need to do though is be able to identify when your opponent is a zombie before you loose a huge chunk of life, then they’ll be free.
Having said that an “I’m going to loose” attitude isn’t great. As others have said, maybe taking a short break would be healthy just to get you out of that mind set. From personal experience taking a break can help a lot and you’ll come back playing better - however you can also play through that slump - end result is if you just play through it you will probably end up better than if you took a break, but taking a break is no play time, playing through it could be quite a bit of play time, your choice =). I tend to just play through slumps and not worry about it too much because I know I’ll come out the other side.
Something else to note is maybe this slump of yours has a cause. I’ve found when I enter a slump it’s normally for an identifiable reason e.g. this may sound a bit stupid and I feel no different, though if I’ve been eating healthily I play a lot better than if I’ve been eating a lot of junk food, drinking coffee also often makes me play worse (even if I can’t feel the effects of the caffeine) and lack of sleep/exhaustion/hunger always make me play badly, as does having something else on my mind. Also I play better when I’m exercising regularly.