I tell my athletes just about every day at practice and especially at track meets: you have to stay positive. Negative thinking only harms their athletic performance even if we, the coaches, think they had a great day. One negative thought can bring you entirely down.
The same is true when I'm working with clients in a gym. If you start thinking things along the lines of "I can't do this" or "This is way too hard" so forth and so on, you'll never accomplish what you set out to do. I'll admit, I've had days where I don't feel like working out or going for a run cause it's raining or it's too cold, but I still do something, even if it's 50 pushups or 25 crunches for each area that targets all areas of my core. And I always feel better afterwards.
As the saying goes, what you do is 90% physical and 10% mental, I believe it 100% accurate. You're body will do what you command it to do, but believing and thinking that you can do something you never thought possible, well, that just gives you that extra something that someone else doesn't have.
Here's the link to the article I read and decided to pass on: Positive Thinking. Hope you enjoy it as much as I did because for me, it just cements what I believe and what I've been telling others for years.