Relationships and OVERTHINKING?

okay im with the guy i liked for a while. we have a lot of fun together and everytime he touches me or holds my hand i get butterflies(: so all is well right? but my stupid mind overthinks EVERYTHING! like i don't know if it is a protection tactic of getting my heart broken but somehow i always tend to talk myself out of relationships. i could be with the perfect guy and find a reason to breakup. How do i stop? sometimes i overthink everything so much that i feel like i cant even enjoy my relationship. i overthink to the point that i make myself think that i do not like him anymore even though i know i do. HELP! i neeed to know if this is normal and how to stop it because at this rate i will never be with ANYONE. i do not think i'll even get married with this being overly picky attitude i have. i want that perfect disney fairytale romance and i might have it but i TALK MYSELF OUT OF EVERYTHING.