You're right, that's a drastic change. But over what period of time did it occur? Was it sudden or gradual over several months or years? It matters because a sudden change is more likely to indicate a problem than a gradual one.
We know that the human sex drive naturally goes through peaks and valleys (pun intended). We also know that people tend to have more sex when they first get together than they do later on. (I mean generally, of course--there are always exceptions.) You could be experiencing either of those with your guy. If you tell me that you were having sex a few times a week in 2009 but now it's only a few times a month, I might say, yeah, that happens. But if it was an overnight change, that's totally different.
If the change was sudden--over less than a month's time--then I'm not sure I believe your boyfriend when he says nothing is wrong. Something has obviously changed. He might not be lying or cheating; something might be wrong and he just doesn't realize it. There are many other reasons that people lose interest in sex that have nothing to do with infidelity. Things like stress, depression, medical conditions, side effects of medication and more can kill the libido.
You two need to have a frank discussion about what's going on and try to figure out the cause. He might need a physical and blood work. He might be okay with less sex, but if you aren't, he needs to know. If it's a problem for one of you, it's a problem for both.
One more thing--why is your sex life the only thing that makes you feel pretty and important? I hope you realize how unhealthy that is. You need to feel pretty and important regardless; your self-esteem should not ebb and flow with the number of times a week he puts his penis inside you. Your self-worth needs to be something that remains strong regardless of external pressures such as how other people treat you.
Yeah, I know: easier said than done, but it's a goal toward which you should strive so that the normal ups and downs of life--diminished frequency of sex, for example--don't automatically make you "feel like total crap."
Thanks for the question.