I hear you. I have the same issues at times thinking, "but that's been done before". But the reality is everything's been pretty much done before in some way, shape or form. The trick is to bring something of yourself to it, something interesting in the way of characters. Characters enliven the plot, but the plots are often simply variations on age-old themes.

The thing is if you think about it too much you'll never write, so it's best to just plunge ahead and do your best. But, yes, I totally have moments like that as well.