Sexual abuse is just like physical abuse, except more damaging emotionally, and if it goes far enough you call it rape, not assault. But it doesn't matter how far it goes-- whether it's kissing, touching, oral, or sex, if he (or she) is doing it, and you don't want it, and you tell him (or her) no, he or she has to stop.
He (or she) controls his (or her) body; you control yours. There's no excuse for him/her to do things against your will. There's nothing you can do to "excite him/her" so much he/she isn't responsible for his own actions. Only your consent-- explicitly saying yes, while not under the influence of drugs-- makes anything ok. It's your choice. If it makes you uncomfortable, or you simply don't want to do it for any reason, you have every right to say no.
Clearly state your limits calmly but firmly. Make sure he or she knows what makes you comfortable. If the person won't stop, protest again, and if he or she continues to push, do what you can to escape. Fight if you can, but don't endanger your life. If you can't get away, and he or she forces sexual contact with you, that's rape. Tell someone. Yes, it's possible that girls can rape guys, even though usually it's the reverse.
If you do escape from him or her, don't say you're going to break up right away. Wait until you're safe at home, tell your parents, and then call for the break up. Don't get caught in a place where he or she could hurt you. People who push hard sexually are often physically violent. Don't become a victim.
Confront your fear, but not directly. Scaring you is the only way an abuser can control you.