if your problem is just being out of shape just practicing movement should be enough to get back in the grove..what you're describing though sounds like you are off in match cause you are trying to focus on your opponent but can't perform properly when looking at a live opponent..pracice keeping your eyes off your character at all times while comboing your opponent in training mode..and definitely do the starman training I recommended.
I definitely sympathize with losing your tech skill from not playing for a while, i merely don't think it realistic when you say that your tech skill is flawless with every other character except your main.. more likely you either don't notice when you miss tech skill when playing other characters
or you just overfocus on tech skill when playing secondaries and are probably doing everything else wrong. I'm just saying your description of your problem seems highly implausible.
even PP's mario has significantly more tech skill errors in matches than when you play his marth, falcon, falco. or at least it did the last time i watched him use his mario against mahone's jiggs.
Anyway, I gave good advice. starman training and not looking at the comp while you practice should fix your problem, unless your actual problem is just being overwhelmed by better opponents. but assuming near equal skill level i think the two things i suggested should help..
what you are describing still seems weird though. I've had days where tech skill was off before after not playing for a while, but 6-8 hrs of training mode always cured my problems, so I'm wondering if maybe your training itself is not being done properly