Think about where you want to be in the future and then imagine how upset you'd be if you were doing something, a job, perhaps, that you didn't like? Now, use that as your motivation to tell yourself that you have to work for it and that it the end it will really be rewarding and you'll be happy that you've achieved your goal. Tell yourself that what you're feeling now will pass (as it usually does) and that you have to do what you must. If you're really down, just set aside 2 minutes to complete a bit of your work and then when it's done, strive to do another 2 minutes until you've completed the work. Tell yourself that you will get there and that whatever you're feeling right now--stress, annoyance, whatever it is, that you will get through it all. Don't tell yourself to work so hard if you don't feel like it, rather, tell yourself to get at least two things done. They don't have to be overly complicated but strive for getting at least two things done, even if they're simple; and when you feel like you're ready to do more, then do a little more than last time and eventually you'll find that it wasn't so bad after all. That what you're doing now is for your future, that the hardship will all pass and in the end you can have as much fun as you want.