Confidence

I have been writing music for almost four years now.  Some of the songs are better than others, but none of them will ever be big hits.  What do I do when I decide that I am just mediocre at this?  Should I quit?  Should I keep doing it, until it isn't fun?  Should I push myself to try to get better?  How can I tell what it good?  Should I be the audience or should I judge the work by finding more people to be my audience? 

I am feeling a bit discouraged.