Need relationship advice please!?

I am 24 yr old male.I have been through many tradegies over the past 4 years-from losing my mother, my grandfather and almost my life in a bad car accident...The problem i am having is I cant seem to find happiness in my long term relationship...I met my g/f 3 years ago and was head over heals for her. She is all I can ask for in a g/f and its frustrating to me that I have such a short fuse with her. I find her attractive, y, smart -everyhing a man would want in a gril but I cant seem to find an attachment. It is the most diffcitult thing in the world. Put youself in my shoes and think how it feelis to have feelings for some one so trong one day and be cold and anit- everything the next. I am in grad school full time and have a full time jon. I am on 50 mg of zoloft weening my self down from 100mg. I stoped going to therapy as these obessive negative thoughts come and go--i found it to helpful in someways and annoying in others. I wish I could jus be happy and Im tired of asking myself if she is the one-- it doenst get any better--- I had many other girls and the pieces never fit...She is beautiful and my mind downplays everything about her. I see her everyday which may be bad but then if I dont want to see her I over think it and tell myself I dont like her...Its crazy...what are normal feelings? I lost them through lifes rough path.....Please help...