Why can't i stop thinking abut her?

hi, i'm a and have been having feeling for this girl i use to work with, it's been a year and a half since i first laid eyes on her and i can't seem to stop thinking about her.i don't know it she's a or not and we really never even talked, just say hi to each other that's just about it. i can't think about anything else but her, i feel like i can't even go on with my life right now because i want to be with her so bad. i think about making love to her, even about the way i want to propose to her. i keep thinking something is seriously wrong with me because of the way i feel about her . i don't even look at other woman because i'll feel like i'm being untrue for the feelings i have for her. deep down inside i don't want to get over her but i need to in order to move on with my life. i guess i just need to know what should i do about the way i feel?