Oh lovely, I hear you. I feel just the same and there's nothing worse, it takes the pleasure out of everything, doesn't it?
Just like you, when I'm good, I'm really good and I feel optimistic and positive and almost like I can see the new me further down the line. Then it all goes awry, and before i know I'm eating Creme Eggs like they're going out of fashion and have undone all the good work and positivity and I loathe myself for being so weak.
I've given myself a strict talking to and I'm determined that that's it. No more. I cannot spend the rest of my life on this endless yoyo of good, bad, good, bad. It's turning everything that is multi-coloured a miserable grey and I've just had enough.
I read something the other day that really struck a chord. Place two chairs in front of you. Look at the one on the left, and imagine that you're sat there in three years, having done nothing to change. You're as big, as unhappy and have had a life half-lived because of it. Now look at the chair on the right, imagine you've made the changes and imagine how your life is now. Imagine how you look and how you feel. Now go and sit on the right-hand chair - visualise yourself further down the line having made a commitment and sticking to it. It's working for me for now, give it a whirl.
Good luck, and trust me, you're not alone.