What you are describing is abuse. There was a power difference in your relationship and your cousin took advantage of it. He was older and knew what he was doing. He knew he could use bribes and your desire to please him to get you to do something he knew was wrong. At 11, you were just beginning to have any understanding about sex and boundaries. This situation would never be defined as consensual because you were never at the age of consent. Research indicates that sexual assaults of children under the age of 12 are most commonly committed by adolescents who are 14 years of age. Please be assured you are not abnormal. Your trust was violated by someone you looked up to and thought you loved.
Yes. It probably has something to do with current problems. Your introduction to sex and intimacy was shadowed by secrecy, bribes, and the sense that something was wrong. It would not at all be unusual to have some of these same feelings triggered as you try to get intimate with a boyfriend.
I suggest you seek out a mental health therapist to help you with this. You are not crazy. You are troubled. Most therapists know how to treat this kind of situation because many people have similar issues. With some support and practical advice, you can put the events of the past where they belong — in the past — so you can enjoy your present.
I wish you well.