You have to take a lot of things into consideration.
1. How does your family feel about it or how will the feel when they find out? Is it worth causing family problems and or hurting others in your family?
2. are you doing it because you fell in love or are there other reasons involved?
3. do you see this long term? if not you may want to reconsider
4. what happens if you break up? what kinds of problems will it cause
5. what if you get pregnant?
First cousins can marry and date in most states in the US. Third cousins is not even an issue, if thats what you want to do, you are free to do it. Here's a great website with tons of info on cousin couples
. It's not as uncommon as you would think, in fact, in early America, most families married cousins only. The US is one of the only countries now to even have laws against it. I welcome your thoughts, let me know if you want to talk more