When I was in therapy after my accident, I was supposed to swim (read: TRY to swim; it was painful and difficult) for an hour every day. I was good for a few days and then I noted that I started making excuses, I was too busy, I could skip today and do more tomorrow (only tomorrow I had another excuse), I didn't feel so good, the water was too cold, I was behind on doing some task, etc. So, what I did was this: I told myself, I'll just do FIVE minutes - just a token to keep my hand in, so to say - and that way I will have met my obligation a LITTLE bit and I won't feel so much like a complete slacker. After all, five minutes is better than nothing.
With that thought, knowing that I wasn't obligated to a big ordeal, I could get on the suit and head out the back door.
So, I would get in the water for my five minutes and, guess what? After five minutes I would say to myself: "well, I'm here and I've done five minutes, no reason I can't do 10." After ten minutes I'd say to myself: "okay, I've done ten minutes, no reason I can't do 20." After 20 minutes: "I'm only 10 minutes away from HALF an hour, might as well take it to there." And after half an hour: "Gee, I've done half, why not just go the whole way? That wasn't so hard!"
Hopefully, you can get the principle I'm trying to convey here. It's like bargaining with the predator, throwing the wolf a piece of meat to distract him from what you are really doing which is building a history of DOing in spite of all the internal resistance. Once you have a history of DOing, the internal resistance gets weaker and weaker and one day, there is just YOU in charge, finally.
So, if you find yourself slipping back, just commit to doing a little bit of whatever it is now, and you will find that once you get the flow going, the rest may come easier. And, of course, there is always the situation where you ARE unable to complete something and just doing a little bit does keep your hand in!