Why can't I get hard for the girl I like?

When I'm grinding or making out with her I only get semi. I used to get raging boners a couple of months ago, but I can't. I really do find this girl sexually attractive and she's hot, but I'm also really emotionally attached to her since I've gotten to know her better. I always wished to not get a boner when I was around her cause I thought that was embarrassing and I thought I would never last long in sex, but now I wished I had one when the right moment came. What the f*** is wrong with me? And would this girl be frustrated that I can't get a full one from her?