Well, often enough, a lot of panels might end up looking bad (or good) because you've spent a LOT of time working on them. Sometimes, it's best to take a break from working on panels and do other things. That way, you're able to look at them all with a fresh pair of eyes.
If you don't like the panel, ask yourself this -- "would redoing the panel slow down my process?"
Are you working on buffer or are you in the middle of an update? If it's buffer for a story you haven't posted yet, you can choose to redo that panel because no one is "waiting on you" just yet. But if it's an update, you might have to accept that the panel hasn't come out as well as you'd hope and just keep on improving.
But rest assure! Everyone feels that way in some point. Sometimes, the panels didn't come out the way we wanted them to or we just didn't have the skill sets at that time.
But I do recommend this:
1. Take some time away from making panels. Rest your hands and your mind and your eyes so that you can focus on other things. And then come back with a fresh pair of eyes.
2. Have someone you trust look over the work. Have them give their honest feedback.
Hope this helps!