I hesitate to call you a bad player. You're simply a player who needs improvement in those areas. Consistency can be difficult if you're playing a lot of characters, or if you're playing bad characters. I suggest focusing on only one or two, and choose characters regarded as high and top tiers. As tempting as it may be, maining a low tier so you can be a low tier hero is going to have you fighting an uphill battle. Choose your favorite among the better portion of the cast, whether that's in general, or in how they play. If you really do want to main a low tier, it may be best to play a better character for now, as that'll make getting better at the game easier. You can always switch to a low tier once you feel you've improved by a lot. You may need to spend a while getting a feel for some characters until you find one that fits you. I'd recommend training mode and maybe even fighting whatever difficulty of CPU you think you can handle, that way there are no stakes and you won't get swamped by dedicated mains as you try out, say, Mario for the first time.
Resetting netrual is tough, because advantage states are really strong in Ultimate. Remember that drifting away to put yourself offstage so you can recover is an option if you're getting juggled. I just noticed your signature - getting out of disadvantage is probably the hardest with super heavies. I would advise trying out other kinds of characters, as it's usually less rough with them.
As for mindset, play to practice and learn, not to win. Obviously, you should still be aiming to win, but it shouldn't be your primary goal. Save replays of losses, and think about what they can teach you. Think about the little things you could have done better, which may have resulted in a win, and keep them in mind for future games. Try to implement them. For an example, did you lose because you didn't know how your opponent's move(s) worked? Study up on that move. Playing against a CPU might not be a bad idea here either, if it was a seldom-seen character. CPUs aren't the best and can cause you to form bad habits, so fight them sparingly if you can, but they can help you get used to how a character's moves work. Acquiring more game knowledge can really go a long way.
Also remember, there's no shame in losing. Your opponent wanted to win as much as you did, after all. And don't let losing a stock/game tilt you too much. Remember that you're always capable of making a comeback. I've seen many players win games when they had been on the brink of defeat, and I have done so myself.