@FlappyFish sorry to hear it's not going well. What kind of things are you eating? The same as you ate on Mounjaro but more of it or your Mounjaro diet plus treats? Or totally different foods to when you were on MJ?
If you have reintroduced carbs, sugar, more processed foods then they will give you water weight gain. I put on 7lb on my summer holiday as I relaxed my eating a bit. That's normal from a change in diet. It doesn't mean you can't maintain your new post Mounjaro weight.
I know a few people have just rubbished TDEEs as varying a lot but do you have a daily maintenance calorie goal? If you have a sense of how much you should be trying to eat - calories, carbs etc then you can plan a day's food which builds in your protein, carbs, treats but still keeps you at that maintenance calorie level you have chosen. If it turns out you are gaining on that calorie level then you need to cut it a bit.
Without a goal of how many calories to eat you can too easily drift off with portion sizes, too many extras including treats. Track everything that passes your lips, plan your day, learn which healthy foods you find filling, satisfying etc. Good Luck - you CAN do this maintenance thing!