I like this girl alot, what should I do?

From my point of view, she is in depression for the fact you mentioned, about her breaking up with her ex. It is possible that she still harbors a great deal of affection for her ex, and could still be hoping he would someday return to her. If she has openly said that you are her "best guy friend", then she probably have that close-friend affection for you, but isn't ready to accept any possibility of being with you yet. The fact that she still talks to you a lot could mean she is missing the attention from her ex and needs another guy's attention, just not so in the intimate way. She just isn't very ready yet, but it's obvious she has affection for you too. She just needs to let go of her ex and accept a world of possibilities with you, but do not declare too quickly that you like her. Her state right now might not accept serious relationships yet. Take it easy and continue being there for her. Sooner or later she will wake up and realize how much you care for her. Good luck. :)