I'll give you a bit of advise that my first boxing coach gave me back in the day that has never left my mind.
When you are training hard and you want to quit, think, "Is my opponent doing this, is he quitting now?" Every work out you have always think that, and it should motivate you to push that little bit further.
There is no better feeling than knowing you are doing more than your opponent in training. For example, getting up early and running while you imagine that you opponent is still asleep.