What is wrong with me? Need advice

My girlfriend and I have been together for almost 9 months now. Things are pretty serious and they've been some of the best months of my life. I've tried to date other girls in the past but i always found a flaw with them and just stopped talking after a couple of weeks . Until I met my girlfriend now I thought i would never find anyone i could get serious with in this town. But when i met her she flipped my life upside down, she made me happy. I truly believe that she is possibly somebody that i could spend the rest of my life with. But recently i haven't felt the same. I didn't know what it was at first so i tried to just play it off, thinking it was just a phase that would pass by. I started trying to figure out what it was. Did i miss my friends? I tried spending more time with them and all that was on my mind was her. Do we spend too much time together? I asked for a little break from not seeing each other and I broke after 2 days and went to her house. I honestly love the girl, i know i do. But I can't figure what these feeling are. It's like my heart is in the relationship but my head isn't. I keep telling my head to shut up and just listen to my heart. If I love as much as i do and she has made me as happy as i am. Why am I like that. I'm wondering if it's real or am i just going through the motions and telling myself i want to be with her. So I sit down and ask myself...Self? Name 1 reason you don't want to be with this girl. And do you know what i think of? Absolutely nothing! I can't think of a single thing! If i had to sit down with two pieces of paper and name all the reasons i do want to be with her and then all the reason i don't want to be with her, I would have to use the other sheet of paper to finish all the good reasons. My head is still trying to figure out why am i not feeling the same about this relationship. So now I'm at point where i feel like im going to tell her we don't need to be together right now until i figure out whats going on. I need to figure out if this is real. I feel like if we break up for a while and i go out and try to have a good time but all that is on my mind is her then maybe it is. Or if i go out and try to talk to other girls and all i see is her face then maybe she is the one for me. But im so afraid to let her go. What if during the time were apart she starts to lose feelings for me or she is afraid to take me back when i do realize i want to be with her.....UGGGHH i'm a mess