No luck with my fishtank?

I take VERY good care of my pet fish. I feed them by the directions, I clean their tank once a month while doing a 1/3 water change once a week...and all of them died...I got two fish at first, but they were sick at the store so I don't blame the tank. The I got 6 mollies. They all died. Then I got 1 pleco, 1 platy, and two guppies. They died. Then I got a betta. It died. Is this the tank? Or is it just bad luck? I'm temped to through it out or sell it because I want fish to have a good life, and if it's not with me i'm fine with that. So, should I sell this tank or keep it?