If you don't mind me saying this... I think it's also the worry.
You know... you've been out and had a bad experience and got all panicky... and now, well, when you're out you worry about it in advance.
I suggest the following. I don't know when you run, but get yourself all sorted for it. I do the following. I eat some carbs at lunch time. Make sure I drink at least 2 liters during the day before running, have a good loo session as well. Download something interesting as a podcast on my cheap little machine.
I get my running gear out (not too warm), get my watch ready and just before I set out, I take a deep breath and say: I'll just run for a laugh. Listen to my interesting podcast and just simply enjoy myself. I sent off really slowly and then just let my legs do the work. Switch off my brain.
Often, when you have a crap run, one finds it hard to forget about it. And i might suggest, that the whole "not being able to take a full breath" is an anxiety thing. If you stopped and did it, it would work. I think you're panicky that you can't breath freely, but hey, you're running... it's exercise.
My advice. Reduce all the ifs and maybes before a run and RELAX. It's not a competition, it's supposed to be fun. The more fun it is, the better you will get at it. Just enjoy yourself. Nice and easy...
Hope this helps without being patronising?