Are mood swings bipolar or depression

I have suffered from depression for a very long time. Currently, I take Lexipro daily and that seems to help. By help I mean that I'm happy, my self esteem level is high, I believe in myself and generally active. However, I get these "bad" days where I feel sad for no good reason; I put myself done and wish other people did the same. On these days, I'm very volatile, it doesn't take very much for someone to set me off and add more fuel to the fire. If you were to ask me if I'm going to like myself tomorrow or not I honestly don't know. I go to bed every night hoping that the tomorrow will be better than today.

Does anyone else have this experience? Is there something that I can do, without the help of others, to fight this and have everyday be a "good" day?