One Step Back
You've begun. You're motivated, and making progress. Finally. Whatever you have been meaning to do for months, years even, it's starting to take shape. Momentum will come and soon what used to be hard will now be a regular habit. Confidence will stir and you feel invincible.
Only inevitably there will be a setback. Something will happen, likely out of your control, that causes progress to stop. Possibly even regress. Those moments can be very discouraging, even worse than when you did nothing. But it's in those moments that your long term success will be decided.
In that temporary moment of failure, you may feel justified throwing in the towel. I tried my absolute hardest, did everything I could, and still failed. Obviously this is not possible for me. But deep down you know that is false. Yes, failure will come, and the step back after taking two forward is frustrating, but it is not an excuse. Look back and see how far you've come, even if it's only been a few days. Then look forward to a year down the road, and the reward you will gain from hanging on.