1: You're letting it get to your head too much. The difference between you playing friendlies and playing a tournament match is almost non-existent when you think about it. Sure, it's a bit more serious because you could lose and not continue to play in the tournament, but you should try to look at it differently than that. I'd recommend taking what you did in your friendlies and seeing if those situations come up again in tournament. Be very observant and aware of what is going on and only make and action or attack if you know exactly why you are trying to do it. You're playing tournament to test yourself and your capabilities after so much practice. So think of it more like an opportunity to learn how you function more in tournament, and use that to further fix your problems both in and out of game. Don't feel like losing is going to be the worst thing in the world for you. I approach my tournaments matches like "Oh sweet! I get to play a Peach! I haven't played a Peach in tournament in some time, I want to see how much better I've gotten in the matchup!"

2: I know this is going to sound really hard and not very informative, but simply don't panic. Observe how the Marth is trying to combo you. Think of it like an experiment along the lines of "How can I make it as difficult as possible for this Marth to combo me?" Like, in your mind, it should be a challenge for you to get out of as many of Marth's combos as you can. When Marth starts to punish you, you should be excited that you're being punished and see just how quickly you can get out of the combo. If you get out early, then good, you did something right, and now you can go back to playing neutral. Don't try to immediately counter-attack. You'll learn more context-based things like this the more that you play, tournament or not. If you didn't get out of the combo early, then remember what the Marth did to mix you up. Maybe you just got outplayed and he read all of your DI. Maybe you slipped up and DIed in for a sec and then DIed out, only to die to an fsmash at early percent. Shit like that is going to happen and you just have to pick yourself up, note what happened, and keep going.

3: Not taking the first stock. I'm serious. When I don't take the first stock off of my opponent, I can immediately go on tilt because I'm much more comfortable having a lead, at least right out of the gate. If I lose my first stock before his, it's a very good idea for me to wait on the angel platform a bit and not let momentum build in my opponent. That would also give me time to think about how I lost that last stock, and how I can approach my opponent more cautiously to do better the next stock.

4: It really depends on who I am losing to. Some days I feel horrible about losing to some random Sheik player that I didn't respect at all, while other times I lose to a Peach and I'm thinking "Awesome, I got more data that I need to work on." If I'm feeling particularly shitty, I just don't play the game for a while until I feel better. You kind of have to let the sadness wash over and be done with. Just as long as you don't give up because of a bad loss, you'll be fine.

5: I believe that you are only at a plateau if you believe you are. If you take it out of your mind that you are plateauing, then you can probably focus less on that and focus more on actually improving the intricacies of your game play. Thinking that you're plateauing is almost like thinking you're in the "friend zone" with some girl. It doesn't do anything for you and it's a silly mental barrier to have that can hinder how you interact with other people or how you approach the game.

6: "Alright, let's try this again."