Depression

Its back!! Sometimes I wonder if I will ever just be a happy person. Just when I think I can't be any happier, I hit these low spots. I am not talking just a few hours, it always ends up being DAYS that I just don't want to do anything or talk to anyone. I just want to crawl up in a hole and cry for hours and hours! It is really putting a toll on my relationship with Drew. I always end up taking it out on him and then we aren't "normal" for the next week or so. I don't know how to fix it. I want to be the best girlfriend I can be to him, but it's like it is not in me to do so. Is this possible? I don't want to cause him unhappiness. Should I let him go? I am not sure I could do that. I love him too much. I have tried to let him go several times and always end up right back where I started. Leaving is not the answer to my problems, but is it the answer to his happiness? I have no doubt he loves me, but does he love me enough to stick through my depression times? He has said before depression is all in my head. I think this is why I can't talk to him when I get depressed. He doesn't understand what it is like!! He is suppposed to be my best friend but yet I can't talk to him about my biggest problem. Is this a healthy relationship? Will it last a lifetime? I don't know the answer to this. I may not EVER know the answer to this. All I know is that I DO love him with all of my heart and I will just have to pray that God can help me overcome this.