When feeling a little low, or if in a dark place, then step one: know and accept with certainty that it will pass. If you need motivation to move beyond that dark place then (for me personally) it helps to visualise positive scenarios from the past.
This might be things such as passing exams, getting a job or a successful social event I attended. Winning at life in other words!!
Visualise yourself in these situations. Realise that you can create opportunities like these again and continue to strive for positive situations as much as possible. Visualise yourself in these scenes and it can help boost your mood and more importantly you begin to feel more in control of negative feelings.