I'm honestly kind of annoyed with myself. For as long as I can remember, with the slight exception of my last ex, I always become numb when I like someone. I'll like them on and off. I might be really interested in them at first, or become interested later. But either way, my feelings always shut off and I feel like I don't like them anymore. Then the feelings usually come back. I can't stand my own inconsistencies. It makes dating really complicated and difficult. =/ Is this normal?

Don't think about it too much though. Just take things as they come, and never fight back your feelings or try to suppress or ignore them. Come to terms with them, and try to figure out why you're feeling the way you are. If you don't then those feelings of uncertainty will escalate even more.
A lot of people refer to that as burning out, or death in the passion of a relationship. It's perfectly normal and, in fact, many marriages and good relationships end when this passion burns out and suddenly one or both members lose interest in eachother.
The trick to getting around this is to find someone that is interesting enough to keep you interested in them. Someone who has fun hobbies, intellectual interests or just has an interesting personality in general to keep the relationship from being stale. Some people need excitement in their love lives and you are probably one of those people.
My advice to you is to keep looking for someone that can keep you interested. The guy that you shut down may be perfect for some other girl, but not for you and that's ok.