When you're feeling down about your art, try to remember why you drew when you were a kid...for fun. To get it out of your system. We all have bad times, but don't forget why you liked drawing in the first place!
However, personally I believe if art is absolutely nothing but stress and you can no longer enjoy it, then maybe you should consider doing something else. That's not a bash or meant to push people to give up once in doubt by the way, I really mean if you're nothing but miserable in making art, then either take a long break from it or move on to something else for the sake of your mental health! Don't do anything you can't stand.
Also, consider what you're working on and whether you still like that anymore. Don't feel trapped by your own comic. <-- That made me miserable for a long time.
Improving, getting popular and and all that stuff is great and all but don't forget to enjoy it! You can't enjoy it if you worry about sucking all the time.