Mentally tough?

I was scheduled to run three miles today, and even though I was totally exhausted I still showed up for the run. I did three miles on Tuesday and it was a breeze, but I gave blood on Wednesday and was totally lazy on Thursday. So, when I ran today my legs were sore almost immediately and I was just not feeling it. BUT, I need to start "feeling it" pretty quickly if I'm going to run this half-marathon.

So, the question is: How mentally tough am I? Am I going to spend part of every afternoon alternating between telling myself "I can't do this" and being surprised when I do? At what point will I be able to believe in myself enough to get through a run? I ran four times this week, between 2-3 miles each time. That's awesome, especially for the first week of training. But I'm focused on the idea that I "should be doing better".

This attitude is a sign of my overall mentality toward myself--I never feel like anything I do is enough. I always feel as though I could do more, and even if I do something really well I focus on whatever the weakness was (walking for five minutes, two points on a test) and I use it against myself. I need to get out of that. It's no way to live, and eventually it's going to cost me things that I have earned and have every right to enjoy.

So March's resolution is going to be to not be so hard on myself. To increase my mental toughness---against myself. To focus on being a more balanced critic, celebrating my successes as much as I berate my failures.