TL;DR I'm horny all the time and my boyfriend isn't.

I feel for you. Same thing with me and my wife. She's barely sexual at all. She'll happily go for two or three weeks before implying that we should have sex, by which time I'm ready to have sex with a barn animal or, worse, a crackhead.

I worry that since he knows my sexual history is much more adventurous than his he might think I'm sort of gross, but I'm afraid to bring anything like that up.

I don't think you should worry about this. Hopefully he does not think anything like that. If he does - that is his problem, not yours -you should not be ashamed of anything "adventurous", generally speaking.

He can't cum in my mouth (even tho I ask him to), because he wants the "real thing". He won't go down on me and he isn't interested in any position besides me on top or on my side. I love him and want to fix this but I don't know how.

From my perspective a relationship with you would be sexual nirvana. However, your boyfriend is simply cut from a different cloth than you are. I usually think that some degree of sexual incompatibility can be overcome, but it your case the degree of incompatibility is quite wide. There's nothing wrong with you. There's nothing wrong with your boyfriend. But that doesn't make you right for each other. You may love him, but that doesn't automatically make him the best person for you to settle down with. You may choose to stay with him, but if you do you will have to accept sexual frustration - he may love you with all his heart but he doesn't have much control over his sexual personality at this point, and he'll never be able to sexually reinvent himself to the degree necessary to be more compatible with you, just as you realize you could not do the same to be more compatible with him. To be blunt: the best thing for you would probably be to find a man that is more sexually compatible with you. But it's your choice.