Well, for starters I try to keep in mind that progress is not linear, and everything is in constant flux—feelings, life circumstances, desires, levels of energy, etc. You will never get to a point where things are consistently getting better without going through setbacks, because that's just not how it works. If that's your expectation, you'll be crashing face-first into disappointment every time things go south. The trick, at least for me, is learning to accept that reality. The overall trend should be moving upwards towards the better, but it's not going to be that way every day, just like the bad days won't be there forever. You can't "know" you'll always go back to square one, even if that's been the patter for months or even years.
That being said, you need to look at your specific situation. What causes you to come crashing back down again and again? Is it something in your environment, like a family or school/work situation? Is it financial? Your physical health? Your mental health? Depending on what you're dealing with, there could be things that need to be addressed so you are not constantly falling back on your ass.