It's hormonal. Some people just have the right 'sexual chemistry' to make the sex seem better. The pheremones from certain people will make you more aroused than other people, this makes for better sex. There's also other factors like your partner's body might be more or less attractive than previous partners, and their sexual technique will be different.
So um, good luck. If you decide to break up with your boyfriend don't tell him why. Having a g/f tell you that she's dumping you because the sex was terrible compared with an ex... that would hurt. Just tell the guy your feelings changed or you want to be alone or some ****. Don't insult the man's libido, that's just cold.
Anyway, good luck!
I'm not on any medication, no im not stressed, and nothing is different now from when i was with my ex. I have no health problems that i know of and no i don't think about other guys in that way i reallly do love my boyfriend a lot. We have broken up before and just friends never worked we missed each other way to much.
Try to picture your boyfriend and you being just "friends". If that seems to fit and make sense then that might be the answer. Are you attracted to any other guys? If you are attracted to other guys but not to your boyfriend then maybe you should move on and keep him on as just a friend. If you are not attracted to other guys or anybody at all then the previos post by 'Somewhereovertherainbow' might make more sense.
Are you depressed or on medication? Are you under stress? What circumstances are different now than with your ex? Maybe your hormone levels are out of balance or there is an underlying medical condition you hadn't realised like an underactive thyroid. This can effect sexual enjoyment. Sometimes due to other factors we can find it easier to enjoy sex with those we have no feelings for. this is purely mental and with counselling can be overcome. good luck