Is it normal that i only "come" when masturbating?

I think it's normal but, should be worked on. You should be comfortable with a lover. So much so, you are able to relax, let loose, and really open up and give yourself up to sexual arousal. The previous encounters with your cousin that lasted 6 years could very much be why you hold back your release. Depending on what exactly he did with you as well as whether you believe you began liking it or not (coping mechanism).