Let yourself feel really down.  Curl up in a ball on the ground if you like!  Or just sob if you that's what you feel like doing.  But put a time limit on it.

Say 15 minutes.  

Then when the time's up, take up some activity, like going for a walk, cleaning the house, calling up someone you love or you feel is safe, and so on.

It's important to allow yourself to feel what you're feeling.  

There's a saying: what you resist will persist.

So the easiest way to get through things is to allow them to be.  Drop the resistance.  In other words, let go of the idea of trying to overcome it.  

So just let yourself feel down.  

But again, put a time limit on it that seems reasonable to you.  I usually use 10 - 15 minutes.  (Secret: I usually don't need that much time.  Either I fall asleep and wake up refreshed and feeling better, or something in my mind switches over within a few minutes.  But I attribute that to having gotten good at dropping my resistance to things as they are.)