Sometimes at the beginning of a workout, my mind resists by saying "I'm really tired. I can't do this." (Even though I have quiet easily accomplished the workout before.)
Whenever I give in to such negativity, my muscles DO start to feel heavy and I stop quickly.
However, I've noticed that repeating, "Yes, I like this experience. This is the way I want it. I CAN do this" internally, helps me overcome that resistance and makes me feel much more energized.
This follows from a workshop I took a long time ago. In it, they were talking about how you can stop suffering (and you suffer when you resist) when faced with a difficult decision that you feel is forced upon you. Simply pretend that the decision is actually your choice. Say to yourself, "I choose this" and move along. It was remarkable how something so small can make such a big difference.
And many times I don't even notice that I am resisting something. One good way to notice is: if something takes longer and is harder for you (than for others), there is a good chance that you don't want to do that activity — that is, you are resisting that activity. And of course, "whatever you resist, persists." So the key is to not resist. Never fight the system, or something like that.