I've been trying so hard with him lately.

I've been trying so hard with my fiancé lately. He always try's to find an issue with me to argue about so I've been trying extra hard lately reassuring him that I'm not doing anything etc. it's honestly  draining for me explaining myself all of the time, then I find myself being snappy towards family members because of it. One day he hates me the next day he spends hours telling me how much he loves me. So this morning I woke up and he texts me saying he wants to be done because his friend follows me on Instagram and I told him I didn't know the password, WHICH I DONT. I don't know the password so he just ruined my morning. It sucks like he determines rather I have a good day or not. I love him so much and feel so stupid. He's very insecure in this relationship, I mean I've lied before in the beginning but so has he! but we've decided to start over and he is still picking at me. He never believes me ever. If he asks what I'm doing, I'll say watching tv and he will say prove it by getting me to send him a picture. We've been together for 3 years, it just really sucks. The other day I spent the night at my sisters house and I left some clothes in my car and he came by (without my knowledge) and took my bathing suit and other clothes out of my car at 3 in the morning calling me a cheater. He's been physical with me before I mean nothing serious but he made me feel like if I hadn't lied about what we were talking about he wouldn't have done it. I can't tell him the truth about everything because he gets mad so easily over nothing!There's so many things making me unhappy in this relationship but it's like he has this hold on me and I can't seem to let go. I'm always finding myself reassuring him ALL DAY that I'm not doing everything wrong. It sucks. I wish things were like they were when we first started dating. I want someone to do nice things for me and just be nice to be in general. He's so mean to me yet tells me how much he's in love with me. It's a constant up and down thing with him. I'm 23 and so is he. I know he's still immature and obviously I know I seem to be too, to even put up with this. He's hit me, degraded me, made me feel and look stupid. Yet I can't seem to let go. I find myself being happier explaining myself all day and hoping he will believe me by the end of the day then just trying to call it quits. I love him so much I don't feel like I can let go. I feel stupid and embarrassed even writing this considering I don't tell anyone about my relationship. Sorry for the long lost :(  just really need some advice?