Focus on the solution, not the problem. It's all a matter of perspective-- once you can turn 'why aren't I winning' to the all-consuming thought of 'I will get better', that makes a world of difference. That was the main turning point for me in Pokemon-- where I stopped beating myself up for what happened and eventually started learning to focus on why it happened. Only just reaching the point where I can apply that in smash, of course, but it was really eye-opening to see the skill gap firsthand and then to recognize that, rather than mechanical, the distance was mostly a matter of experience.
Either way, the best solution is just to play, play, and play some more. If you play, and your thoughts are on how to improve, you will get better. Try keeping around some old replays to remind you of that fact-- the more you can cringe at mistakes that you wouldn't make today, the better you are by now. The only benchmark that matters is yourself so long as you dedicate yourself to improvement. The weight of extensive practice behind your play will trivialize any other problems if you just keep adding to it.
Try setting a minimum practice time for each day. Ideally, a few hours rather than one or less. Nothing matters but improvement relative to yourself. If you lost to a Spamus or Flowchart Link last week, then beat them today? If you couldn't touch a half-decent Little Mac at all until you finally take some wins off one today? If you just learned a new combo? If you found some new property or hitbox on a move? No matter how trivial it may seem, just focus on what you're doing better first... Progress is progress, and you can never do anything but improve by practicing.
Hope I was closer to 'encouraging' than 'rambling schitzophrenic', there, at least. ^^"