So your boyfriend cheated on you. The girl is most likely not as pretty as you and your boyfriend say, "It was a mistake!" or "It will never happen again!" You're furious at the girl but you love your boyfriend, so you cut him a bit of slack.
You start blaming yourself and your self-confidence is lower than ever. Let me tell you something. You have no reason to be mad at the girl. If you need to be mad at anyone it's your "boyfriend".
He didn't have enough respect for you to say he didn't want to be with you anymore or had feelings for someone else. He would've rather seen you more hurt and see you become insecure than just break up with you and give you time to heal. It is NOT the girl's responsibility to make sure the boy stays loyal.
It is NOT her job to make sure he stays under control. HE is the one who is responsible for cheating. HE is the one who the blame needs to be on. It's not your fault nor is it the other girl's fault. It's his. No one deserves the pain of being cheated on or even having their emotions played with.
Cheating is a hurt I wouldn't wish upon anyone. You have the right to be mad at the situation and you have every right to not like the other girl, but it's not her fault. Not to mention, if you have to be overprotective about your S.O or text other girls and say, "don't text my boyfriend", then you don't trust your S.O.
You need to be with someone who can have conversations with others of the opposite sex and be confident enough in your relationship that they won't cheat on you. If it gets to the point where they feel like someone is hitting on them, they need to defend your relationship. If they take advantage of that trust, then leave them.
They don't deserve you. Being cheated on hurts. Overthinking and even convincing yourself that your S.O has feelings for someone else, hurts. You have to stay strong. You have to know that it was no one's fault except his own. Don't allow him to take advantage of your trust again.
Don't allow him to hurt you again. If you have to cry to get through the pain, cry. If you have to work all of the time to keep yourself busy and not think about it, try to ask for more hours.
Whatever healthy way you can get over him and the pain, do it. It gets better and let this be an opportunity to gain more confidence and know that you didn't need him. You don't need anyone.