Afraid of commitment need some advice?

Okay well i am in this relationship with my gf and we have been together for almost a year we are both 19 yrs old and well see here is was has been bothering me everday for a long time now . See i love her dont get me wrong its just everytime i think about us living together or us getting married in the future i just kinda freak out a lil and get kinda slightly depressed i start thinking like ooo im gonna be with just one woman for the rest of my life do i really wanna do this in my future . An see she also has some problems but i still love her such as she has a messed up druggy / disfuntional family , she was d in her past when she was a kid , she has depression daily due to her birth control shot she takes ervery 3 months , she is biual that part just bothers me to because i think she will leave me or have an affair with some other person , when she clearly told me that she only wants to be with me an truly love me , but in the back of my head the thing still bothers me. Sometimes i want to break up and leave her and ust be single but other times im happy im with her . I think that if we break up i will never find anyone else in my lifetime . Am i just a messed up person or is it normal to be afraid of these things anybody please help i need some serious advice