I've slid back myself, dust off and go again. Now you not only know that you can do it, but you also now know how to do it. The big thing for me all my life (other than the plump, stout, fluffies) was I had no idea where to start, or how to get healthier. Now I do, and so do you.
Also remember that you may have slid back one step, but first you took four steps forward. You really are a long way from where you started, even after slipping, just stop the slide now and you will do great.