They often say it is normal to remember your first and sometimes people do take this a little to the extreme. My ex was EXACTLY the same. He said no-one can live up to the fantasy he has of her, the way she looked, the way she moved, the way she made him feel etc. His problem went on for years and he eventually needed therapy coz he couldn't enjoy sex with someone else in the same capacity as he did with her as he idealised her. It was psychological for him, something about her providing him with his first sexual experience after years of being single an masturbating, and providing him with pleasure which he inadvertantly and subconsciously related to her, therefore he went on to 'think' he could only have good sex with someone that looked like her. Really destructive for him. He is 31 though and had this first sexual experience in his early 20's (quite late) so it is good you want to nip this problem in the bud before you get to the stage where it has a detrimental effect on your sexual health and relationships. I have no advice to add like I said he ended up having to see a therapist but I don't know if you need to go to that extreme. Although being a mind thing it may assist. Good luck.
Some people are drains and some are radiators... Keep clear of the drains and hug the radiators!