Well, this is definitely something I'd suggest talking to your doctor about. More than likely it is not a physical problem so much as psychological. But it still warrants a look by your doc. As Anniina said, many prescription drugs can affect a person's libido in such a way. There are also other physical issues that could be contributing to your difficulties.. best to talk to your doc about it. But, since you do admit feeling a mental block, chances are it's more psychological. I went through a major depressive episode where my normally high sex drive absolutley died... even to the point of disgust if sex was even mentioned. Now, it sounds to me like you want to want sex. I am not a psychologist, but I do enjoy studying psychology, and from what I know there are at least 2 possible disorders with these symptoms; female sexual arousal disorder and hypoactive sexual desire disorder. Again, if you speak with your doctor, he or she may refer you to a psychologist or psychiatrist specializing in sexual disorders. Believe me, it's not an uncommon thing, don't be ashamed of it. For myself, I went to hypnotherapy for both my depression and my dead libido. Both were very quickly resolved after hypnosis. It may be something you'd like to look into. Not only does it work like conventional counseling in a faster way, it also usually leaves you feeling refreshed and relaxed for a while after each session.
The most important thing is to not beat yourself up about it. This kind of thing happens, even to the best of sex drives :)
As for your new boyfriend, assure him it is not his fault, and that you very much want to stay with him (if you do, of course) and ask for him to stick with you as you work to correct this.