You seem to be setting high expectations for yourself, which is something that I believe that you should just let go. I would personally say to just go easy on the expectations, and have small goals that you know that you can reach. Setting the bar up that high will only lead you to frustration because results aren’t meeting your expectations. You also have to take into the account that your mentality influences how well you do, and how well you perceive things. Having a negative mindset will only serve to hamper your skill by making you believe that you are worse than how your actually performing.
I wouldn't focus on the online mode of Smash, because it doesn't actually prove how good you are as a player. Taking into the account that the online mode can be laggy with delayed inputs than the clean inputs while playing offline, you shouldn't take the online mode seriously. GSP is also useless, because it only tells you how many players you are better than, but not the total players like a normal leaderboard would tell you. Most pro players who play online don't actually care about their GSP, so I wouldn't really worry about it too much. Even if you're not able to reach elite smash, it's still okay.
You don't have to tell yourself that the losses are "only temporarily", and when you set high expectations for yourself, you are only setting yourself up for frustration. It influences your negative mentality, and makes you believe that no matter how much training, practice, or labbing you do, you aren't improving as a player when that is not true at all. Right now, my main focus for you would be to take your losses in matches as a learning experience rather than a complete defeat. Taking your matches as a learning experience will help better you as a player, since it will tell you where you excel and where your weaknesses are. Just because you lose a match does not mean that it represents your true ability as a player, since sometimes opponents can outplay you.
One thing I noticed about your post is that you show frustration and anger when you are losing, and your previous frustration is constantly over your shoulders. Mentality is a very important factor that can influence how you perform in games. Since you have a negative mindset, it is tampering with your ability to play and enjoy Ultimate. I'm not sure if you have anger issues or not, but the first step I would recommend for you to do is to figure out a way to stay cool after matches. If you do have anger issues, it is recommended for you to acknowledge the fact that you have anger issues. I know it sounds ridiculous but there are people out there who won't admit or face their own problems. There are also people out there who are delusional, and it would be bad if you end up being 'that guy' who believes that he's the best player in the world. Please don't do that since that would only just add more frustration over your shoulders. If you do have anger issues, try to take a few breaths or so after the match to calm yourself down, or take a break after each match or so and come back later to calm yourself. Since this is a wide problem that's not only apparent in the gaming world, there will be solutions or steps that will be different among people to help alleviate their anger issues. Honestly I would say to just try out different methods to see what works out the best.
It's also worth acknowledging that sometimes a break off of the game for a while can help you out more than you think it does. You don't have to quit or force yourself to continue the game if you feel like it torments you and brings out the worst of your mentality. Sometimes we all need breaks and there's nothing wrong with that, although if you choose to take this route, you also have to figure out ways to keep your mentality at sake (positive mentality > negative mentality). Simply just taking a break to come back, and not acknowledging ways to maintain a positive mindset will only make the break feel like it was a pointless choice, simply delaying the inevitable frustration. I hope you consider these tips, since mentality really does play a huge factor on how you feel, and play the game.
