I can tell that this distresses you a good deal. Unfortunately, I can’t tell you what you can’t tell yourself. If you were 12 years old, your cousin was 10 and if you were 9, he was only 7. He may have thought he was doing kind of normal “I’ll show you mine if you show me yours” stuff, not realizing that it was much more scary to you. If he is much bigger and stronger and you weren’t a willing participant; if you were coerced or forced in any way – then it’s a kind of assault even though the perpetrator was only 7 – 10. If, on the other hand, you were kind of interested then but regret it now, it’s unfair to assign him the role of an adult rapist. Complicating things further is the need to understand what is going on in the present. If your cousin is someone who molests women, then your experience was among the first of many and he needs to be stopped. But if what happened occurred in a time of childhood exploration, then you don’t want to give him a reputation as a molester that he doesn’t deserve.
Whichever is the case, my guess is that you are more sensitive than most people and that things bother you a lot. You’re also at an age (18) when sexuality is becoming a bigger part of your life and your thinking. You are probably trying to come to terms with what sex means to you. If you are at all scared about it (and most young people are at least a little bit scared, no matter what they may be telling you), it could be that the old experience with your cousin is being mixed up with the current anxiety and that is amplifying it in your mind. If that’s the case, I would encourage you to focus on your current worries and concerns about sex and sexuality and not distract yourself with trying to unravel something that happened when you and your cousin were kids together.
I’m afraid that’s the best I can do with the little information I have. I could be missing something important, I know. If this answer isn’t satisfying and you continue to be troubled by nightmares, I suggest you go to a few sessions with a counselor who specializes in working with young adults.
You need the peace of mind.
I wish you well.