Act As If
There are plenty of days in your life when the last thing you want to do is train, especially heavy. This is part of being a weightlifter, or any athlete, really. You'll go through periods of fatigue, lack of motivation, self-doubt, and all those fun things that keep most people from progressing as much as they could.
One of the most important things you can do as an athlete day to day is act as if - act as if there's nothing you'd rather be doing than training, that nothing on earth would make you happier at the moment than squatting heavy, doing a million snatches, or working on your weakest lift.
Just like the body is affected by emotion, emotion is affected by the body. Making yourself smile actually physiologically improves your mood. Acting excited, enthused and energetic will actually improve the physical state of your body. You can literally fake your body into performing better.
This is a big part of why training partners, a team, a coach and a good training atmosphere are so helpful. It's a lot easier to fake your way into enthusiam if you're surrounded by it. If you aren't lucky enough to train like this, you have to take all the responsibility. All that silly postive self-affirmation, making yourself smile, clapping your hands together to get pumped up to start are all genuinely effective - you just have to convince yourself to do it.
If you don't feel good going into your next training session, just lie your little ass off.