Keep your eyes on the prize, as the saying goes. You say that you will quit in a few months and move in with your girlfriend; would this be something that can get you through the day? If you're allowed to bring framed photos or other things to work, keep a picture of hers (or of you two together) somewhere you can see and keep reminding yourself that in a few months' time, you'll be together again and all this will be a bad dream.

Is there anything positive from your other two jobs you can use to counteract all the stress and negativity that this one is bringing into your life? You didn't mention what they are, are there any career prospects there?

Also - can I just say that I have mad respect for someone who can hold down one job, let alone three... You sound like a strong and determined individual, but I think you've yet to become aware of your full inner strength... Best of luck my friend, stay strong!