For me set backs become a lot more difficult to bear when I am: a) trying to deny my current feelings, and then b) measuring my current emotional state against how I was feeling prior to the set back.
That can quickly lead into a state of mentally beating myself up and making things a whole lot worse by not looking after myself.
So acceptance that this is where I am, and here and now is what I'm experiencing (not last week, last month or last year), and when I can accept it and accept that it's not where I wanted to be, but it is where I am, I can begin to work with that.
That sometimes means looking at what has triggered this state, what the stressors are in my life and being honest about how they are effecting me. Sometimes it means recognising an aspect of a past trauma that I haven't come to terms with, and realising that it is effecting the way I handle things now - and doing the therapy work to help me to come to terms with those feelings and to manage them in relation to the current situation, rather than react to them as in the traumatic situation.