We all feel like we suck sometimes--it's just how it is. But trust me when I say nothing will slow down your writing progress faster than being stuck on the "I'm No Good So Why Bother" carousel.
Solution? Remind yourself of how much you have grown, because no matter how long you've been writing, I can grantee you have. It may be difficult to see how far you have come since change happens so gradually, but go back and re-read some of your old work--and I mean the unedited, right out of the gate stuff that you may have almost forgotten about--and you'll realize the strides you have made. Don't get me wrong, it's painful. Particularly if you originally thought it was awesome and gave it to other people to read, or *gasp* actually queried with it! But cringe-inducing as it can be, you owe it to yourself to truly appreciate how far you have come. It's easy to remember the start and the finish, but don't forget about the road that got you there--potholes, speed bumps, detours, and all.