People who are consistent are usually the ones who achieve their goals. If you quit it would make sense you go on a yoyo diet. A lot of the people here try to make it a healthy lifestyle instead of thinking “I’ll only do this for a set amount of time”.
I would say that being mentally prepared is half the battle. As you can read throughout most of the posts on here, almost everyone struggles throughout their journey and a lot of them stay determined to be consistent or adjust to what works for them—not revert back entirely to old habits.
Rather than sticking to OMAD, have you tried to go on a 16:8 or 18:6 routine?
Or something else that I’ve personally done is a pseudo OMAD where I’d just drink a protein shake for lunch to get some nutrition and then my “one meal” at dinner time.
Some more details would help so we can gauge what 90kg means. It’s common to see some quick initial gains and then it gets tougher.
It sounds like your metabolism is slowing down, for whatever reason, and you’re suffering from low glucose. You could see results from switching to a more ketogenic diet, and maybe from longer fasts. If you can do more exercise that also might help.
When you approach that point of feeling like shit, you could try maintaining your weight, temporarily or permanently. Instead of throwing your progress away, slowly start eating more and/or making your eating window longer. Maybe your body doesn’t want to go below that weight at the present moment - losing weight can be stressful for the body after all.