Pushing through mental blocks

Hi everyone! I need your help. I have a 5k planned for this Sunday. This one specifically if you're interested in stalking me.

Anyway, I am struggling with mental blocks. I can't seem to run a full 3.1 miles. I always have to give myself time for "walking breaks." I don't understand myself. I can run relatively quickly for a quarter mile but then have to start walking. I'd love to run an entire race at a 12 min pace but my body does not seem to agree. It wants to run an 8 min pace for 30 seconds, walk for 1 minute, run again, walk, rinse, repeat. It's totally a mental thing. I struggle to push through and just run!

I burn a decent amount of calories while running. I assume this has to do with my weight. I'm about 5'6 and 30 pounds over my "ideal" weight. It sucks. I work out 5-6 times a week, eat decently (but no where near perfect) and I sleep 8 hours a night. I know all of you new mothers want to slap me. I don't understand why I'm not losing weight. Running definitely hasn't helped me in that department. This is why I decided to try out Body for Life. I'm hoping the extra protein and HIIT workouts will help.

I'm just wondering if burning 500-800 calories during a fairly short run (3-5 miles) is normal at all. I'm beginning to wonder if my heart rate monitor is crazy. How many calories would I burn if I just ran the entire time without any walking breaks?

So I'm going to try to run today without stopping to walk. I have to go on the treadmill since it's rainy and wet out. Wish me luck!

And please give me some advice if you have struggled with this before!