You haven't come to re-evaluate Mario on a skill level that you have either been playing against, or what you have achieved with other characters. That if you constantly have a higher win % with other characters, yet your drawn back to Mario, you have to break old styles and work from the ground up to love playing him again.
I would suggest you focus on the very worst moves first:
-an air move that you don't use, or use the least. I like to practice N-airs and B-airs with mario, then work on D-airs, finally adding in U-airs again.
- a tilt that you haven't found a use for. Might I suggest using his U-tilt instead of his Up-smash for a week or so......
- a slam that doesn't get used... maybe switching the normal time when You would use the D-slam.... stuff like that.
Then make sure you have a maximum range for the Down+B tornado.........
Then add back in FireBalls and Up+B (Up+B back to the stage.... but don't use it normally as a counter til your done).
Once you take the time to see all of the angles you didn't work with before, or saw as weak/useless in general combat, you start to see the full character again. I feel that I get in a slump whenever I begin to use one move-set or combo pattern. Stuff like that brings me down too....... so I do this training...... And normally walk away with a few fun moments, and get stronger with what ever character I was working with. I once did a match with Luigi where I had to start every combo with either a B-air or a Dash-A, I came to love the 1000 slaps of Luigi and the 3 times I know its right to use it. Hopefully this help you out, and put some spin on your slump. Peace.