What is with all these depressive self-help threads lately. First of all, they aren't going to help you accomplish anything, because the advice is going to be nothing but stuff you already know. Play more. Read about your character more. Do more. Blah blah blah. These are natural and logical conclusions you can draw yourself.
Let's take a minute and talk about fun. What is fun? Smash Bros. is fun. This means that even losing, theoretically, should also be fun. Maybe not the actual losing part, but process should still be fun because you had to play Smash Bros at all to even lose in the first place. If you put all of the weight on winning for Smash Bros. to be fun, then you have basically begun to absolutely miss the point of Smash Bros. as a whole. The game is meant to be fun to play, and more fun to win. If losing throws you in to some kind of downward spiral of depression then you need to either take a long break and/or re-evaluate why you even play.
If you're trying to be the new GOAT, I can see how losing might be frustrating, but that's an issue of a flawed perspective. You need to evaluate reasonable goals from how often you are able to play, and how intensely you are able to play when you do. You need to evaluate what it means to you to play Smash Bros.
Once you properly do all these things, only then will you be able to have fun, and then you'll find you'll not be so inclined to make threads like these. I'll be my own example, since I don't want to end this sounding so sharp.
I am never going to be a top player. I absolutely will never be there as life will not allow it. I can only play for at most an hour a day after work (assuming wife isn't showing signs of aggravation from lack of attention), and at most two hours on any weekend day. I can play as intensely as I want but at the end of the day, I simply do not have the lifestyle to become a great player. Now, I have great natural Smash instincts as I did play Smash obsessively during my younger years, so I have experience and a quick, adaptive instinct to keep me afloat, and I slowly allow myself to improve even by the tiniest bit, but at the end of the day I will find myself losing. Sometimes often or repetitiously. It doesn't make me mad because if I'm beaten, that person has put in the time or honed their natural skills enough to allow themselves to best me by being better than I am. They deserve to beat me. I am not owed any sort of win. The fun then becomes in surmounting the next challenging player. Improving just enough so I can beat that player tier that's hanging just above my current level. Stagnation is not a concept that I even take in to account, it's simply another stop on the road towards the never-ending amazing experience of Smash. I always have something to work towards, no matter when I get the time to play.
Try looking at it like this. So you lose a lot? So what? You're playing Smash. If you feel that you need to win more often in order to enjoy it, I think you're just missing the point.