It’s okay not to like the character of Monica Geller. She is surely the least likable one for me. I have my own few reasons that may help you a bit (well of course for hating her!)

1. Although i liked Monica in the initial 2/3 seasons. But she is really the domnating and bossy character that have grown into an ultimate control freak over the few years. Her nature to make everything perfect makes her look like an obsessive woman. It was so irritating watching her getting crazy over her breakup with Richard!
2. She is extremely selfish at most of the time. Now we all know she let Rachel share the apartment in the very first episode although she was not invited but let’s not also forget how mean she can be sometimes. She planned to spend all of chandler’s money for their wedding and the worst part was to exclude him from almost everything of the ceremony! Seriously woman? She even changed the plates that chandler picked for the ceremony
3. .When it came to move to Tulsa with chandler for his relocation she initially agreed but as soon as she got another job she took it and stayed back.I mean she would freak out if the situation was vice versa. She rather made chandler go back and forth instead of moving with him.
4. All of her craziness for getting pregnant that made them both do ‘it’ in a hospital room while her other friend is about to give birth a child. How horny they both could be! At least wait till you go home! Again,tricking chandler to get into bed cause she was OVULATING was really sick of her. If that’s not selfishness on another level idk what is!
5. She opened the wedding gifts all by herself. Seriously she could have included chandler at least in this part of wedding.
6. She constantly bosses chandler as if he was just another friend of the gang. Come on, you definitely didn’t marry him to make him do whatever and however she wanted. She always tries to insult chandler in a humorous way but ends up solving things saying “ oh honey,you i love you and that’s all that matters and BAM! A smooch we get to see
7. Buying all the lotteries all for yourself? Seriously?
8. She doesn’t even notice chandler got glasses or even if he is not in NY with them when phoebe says she missed getting all 6 together

9. Constantly referring that night as ‘my special night’ instead of ‘our’ as if chandler was nowhere in her own little narcissistic world.

I could add more but damn I already hate her and don’t wish to make it worse. I really hoped Chandler ending up with someone more whole-hearted,free to accept something less self centred and less obsessive.