Leaf, there's some good advice above for you. And your type of character will help determine the best method of addressing the problem. What works for one, may not work for another. CBT is a very useful therapy for some, doesn't sit well with others.
So, depending on your personal view, I'd also suggest that it's worth reflecting on your reasons for running and trying to strip that back to its core and, if you truly find that you enjoy it just for the sake of it, practice in allowing yourself just to enjoy a run, whatever the pace, however you feel on that particular day, and take no notice of times, paces etc. Run without a watch!
I think we can all get caught up in that "I'm not good enough at running" belief and yet, unless you are striving to be one of the very fasties, you're good enough if you are out there and putting one foot in front of the other.
It's not instant, believe me. I've taken several years to finally be comfortable with why and how I want to run now. I still get the feelings that I "should/" be going faster, that I "should" be doing races, that I "should" be fitter. The difference is that, most of the time, I'm able to recognise those feelings and let them drift straight on by and get back to just enjoying the peace and solitude of the run.
Good luck.