Motivation is very personal. I do know how difficult it can be. During my last gym session I spent most of my time thinking 'I can't do this. It's just too hard.' You'd think that after all this time it would have become routine - but sometimes it is, and sometimes it isn't. I don't know why sometimes it is much harder than other times.
When it's hard what I do is just force myself to work through small periods of time. Most of the cardio machines give summaries of your achievements every two minutes, so I just aim for that next milepost, and enjoy that sense of achievement, and carry on. I also tell myself that I have got to do this exercise for another, say, fifteen minutes, and, actually, how long is fifteen minutes? Peanuts, time-wise really.
Quite often, when I am going to the gym I'm thinking 'I don't want to do this today. I don't have to - it's only for me. No one else will mind.' But I just blank it all out: I think of something else, and before I know it, I'm there and it's too late to turn back. I just might as well get on and do it.
Of course, a lot of the time I'm really enthused and happy to do it, and it's easy.