WHILE my ability to be good at the game could be questioned currently, back in the day when I was a grinder I would watch my own gameplay a lot. I also used to watch casts of my games and no matter how big my ego got I would always take outside criticism no matter how harsh it felt (even if in the moment I would be giving backlash).
One mistake I see newbie players make a mistake of is thinking that watching the highest level of gameplay and comparing it to your own is the best way to improve. I think it's practically pointless. You may learn one or two really cool things, but typically in most games (and including tf2) the highest level of play is a different game you're playing at the lower levels. I found myself improving at a much faster pace when I was watching competitive games that were around my skill level. For example when I was an Iron HL medic I was watching steel medic vods. My mindset was it's way easier to compare your gameplay to someone who's considered barely better than you.
Another big thing is a healthy mindset. Every player I've ever seen plateau has always been for the stupidest reasons. "I'm not as good as I used to be" "My class sucks", etc. These players were literally better than they were before, but they always get in their own heads and play awful because of it. This is just an example of don't get in your own head. You're gonna make mistakes and you will have bad days. It is something that probably happens, it WILL happen.
This is a scenario I think is really relatable with a lot of people, from lower level to top level. You are playing absolutely amazing and you're even impressing yourself, then you make one random blunder and suddenly you feel like you aren't performing as well you just were. This causing an avalanche of you just making random mistake after mistake and it kind of demotivates you. This is a prime example of what getting in your own head feels like and the effects of it.
TL;DR Have a healthy mindset and don't get to yourself. Watch gameplay relatable to your own for easier understandings to adapt your gameplay to higher levels.