I don't understand

I don't understand why I can't have another nice day. I feel horrible and terrible for no reason and just cry alot. Sometimes this lasts for a day or a month or a week or an hour. I'm really tired of being like this and I'm really tired of myself.
I keep trying to do things that will help me. But, nothing seems to work for some reason. I keep annoying my boyfriend by just crying by myself and not wanting him near me when I'm like this. I don't want him to get tired of me or to think he has to comfort me every single time. So, I stay to myself most of the time with this.
I'm tired of feeling terrible just about everyday.