KH9
Why do I play terribly in-games?

Congratulations! Persistence pays the patient player.
In slow games, always first scan the board for "situational details", things like loose pieces (both sides), pieces under attack (both sides), pieces in line with the king (potential pins), outpost squares, is a bishop/knight good or bad, and so forth. The stuff that makes up the raw material for tactics and plans (i.e. my bad knight would be a beast on that outpost square), and then look for tactical opportunities, and weigh them against your idea (is it better to win that pawn or improve my bad knight?). If you can't see a way to implement your idea at this time, you probably have identified why, so see if you can solve that problem first. After doing that, you can re-evaluate if your previous idea is still the best one. You don't need to look for mate from move 1, you just need to be constantly looking for how to make your position better than it is. If you can tip the balance to your favour sufficiently, then eventually a win will come into view on the horizon, and you aim for that.

Gamer rage is the casual term used by the gaming community for the extreme anger that you might experience while playing video games. There are quite a few causes for gamer rage, including a broken internet connection, losing a match in a competitive multiplayer game, or getting into a fight over voice chat. There are also more specific and personal motivations for playing games. Some people may be motivated by the chance to break away from social norms and their own identity and act as another character.
https://human-benchmark.org/

ha ha! Despite my comments on being "situationally aware", I played a game this morning where, being comfortably ahead I confidently moved my rook, thinking the square was defended or that their rook was pinned (I forget why I thought I could do it now), but ooops! neither was the case. I'm not even sure how I mis-read the board, but it was a lovely blunder as they confidently took my rook with their, very clearly, not pinned one.
I don't do any classes or coaching or anything like that, so I can understand why I'm really, really bad, but I enjoy the game. Does anyone have any quick, easy tips for beginners?
Play the bots and try the puzzles.
Thanks, I have tried the bots and beaten a few.
take center, develop pieces, castle, consider tempo, think before you move, learn basic tactics and try to find them in game, always think about your opponents last move, analyze your games afterwards, play longer time controls, learn basic mating patterns (queen and pawn, two rooks, etc.)