Any advice on how to be less mentally lazy, impulsive and dumb?

I think psychology is a significant factor. You have to believe you can easily annihilate your opponent if you put your mind into it. It can also have something to do with feeling underappreciated in the past for things you put effort into. Other than that, stress can make you feel like you just want to finish the task to attend other matters, and you might not know that that is happening in your subconscious.
Also cultivation of habit is good, keep doing it and it will become easier to not be mentally lazy. Just push yourself into doing it the first few times.
It could be that you like to be good at chess but you don't like chess as much as liking to be good at chess, so that will make your mind change its stance once the work is between its hand and it has to work.
Things could be a lot simpler too, like the exposure to this amount of puzzles and drills and theory and complex analysis and high Elo games etc is, without you noticing, intimidating your mind when it sees the amount of work it has to do to be on a relevant level.
If it was the latter, then I suggest you find beginner friends (chances are most of your friends knwo the rules of chess, but never play it. Tell them to play with you) and play with them on a physical board and do all kinds of stupid moves on the board. Stop exposing yourself to intimidating high level chess, just play with beginners and do the most stupid shit and have your own little chess world for a while, and move from there when you feel bigger. Teach someone chess to be your partner. Actually, do it with a few people.
TLDR: Initially, do it for fun and don't take it seriously at all.

So as a beginner player myself, take this with a pinch of salt, but my answer is simple in essence but harder in execution. Don't be lazy, now this isn't just on the board but also off it, Chess is partly a game about logic but also a game about knowledge, as a result it is important to educate yourself on different concepts and positions using books, articles, videos, self analysis and pre-existing games. For me the best way to learn isn't running Chess drills or puzzles, but analysing my own games (mostly losses I have sustained), in depth and with thought. don't do this right after the game, do it a day or two later when you have settled from a loss and you aren't under pressure from trying to win, don't just look at the strategic options, but also consider your mind set, what were you thinking at the time? what was your motivation? did you properly consider your move or fire blindly into the dark? What other lines did you consider?are you working on developing better Queen pawn openings like I am, or something else? All of this should be included in your annotations, doubly so if you are thinking of having someone else analyse your game so they know where your head was at and can suggest different ways of thinking.
I have lost games from being over confident and so I didn't pay attention to my opponents moves properly, or I played when I was bored and wasn't focused, Chess is as much a mental game as it is a strategic one. Make sure you are in the head space to play
The real work is the analysis, I can spend up to 6 hour analysing a game that took me 30 to 40 minutes to play, and often I see moves or mistakes that I didn't see at the time, and this makes me aware of those options or holes in my game for future games. game analysis forum is great as well.
Like anything it takes work to improve, there is no such thing as a lazy GM, and nor should there be.