Maybe I need help...

I've been crying my eyes out tonight.. and I honestly I spend many nights crying like this. My baby is 4 mos old, and tonight I went to put her bed and she wasn't having it. So, I try to rock her and give her a passi and she starts throwing an awful fit and screams like I'm murdering her. Nothing I did would work, and then I had to undress her because she was getting so hot from screaming so hard... I couldn't do anything but cry because I don't know what to do... and I'm all alone. There are so many nights/days like this and it's breaking me apart. I feel like I'm losing my sanity... doing this alone is the worst thing imaginable. I have an exam in the morning and now I'm so distraught I can't focus to study and I'm just know I'm going to fail. What am I doing wrong????? This makes me feel like I'm a bad mother and I can't comfort her. This whole experience alone has made me never want to do this again and that makes me even more depressed. Sorry I just needed to vent.