Frustration only clouds the mind. Think of other ways you can improve. Try sprinting up steep hills, jogging with a weighted vest, jump rope with weight straps on your feet; mix it up, confuse the muscles so they can improve.
I know I had the same problem, I wasn't a runner, I didn't like it. My first IST was 12:30+ mile 1/2 and it stayed like that for 4 pool functions. I was approached by my recruiter one day, he said to me that I'm on the watch list. If I didn't improve I would be getting discharged from the DEP.
I got my act together and I tried many things to improve my run, I had friend that were in Cross Country and Soccer to help me. My next pool function I ran a 10:47, I remember that run like it was yesterday. My hard work payed off.
Just keep your head up and just tell your self, how bad do you really want it?