I want my ex husband who I cheated on with his bestfriend to want me back
That feels very messed up. Why do you want him to want you back after you've cheated on him?
Because I fucked him up emotionally.
Cheating on your partner is one of the worst things you can do in a relationship. The fact that you want to get back with your partner after cheating on them feels very selfish as you may try to them more if they ever have a relationship again with you in the future.
I didn't think that cheating on your partner can be that bad. He doesn't know what I did.
I do not think it is wise of you to downplay cheating, as feelings and emotions are hurt if you do so, and it should be said that you should always consider the feelings of others before you act on something, specially if they are very close to you. You wouldn't want someone to constantly do something that is harming you emotionally, would you?