When Enthusiasm Fades Away

I am dealing with trying to learn why each time I set up a goal to get toned and in shape, I find myself few sessions in with no enthusiasm.
When I set up my goal, my weekly exercise detailed plan, I know that physically I am capable of doing it. I don’t set it to be too high to defeat myself, I truly start small with something that I know physically is totally doable for me.

And when I figure out the plan and schedule it, I get very excited. I have thoughts like, ‘This is great!’ ‘I am going to finally do this!’ ‘I’m going after the body of my dreams!’
And when I start few days, I get all excited about myself, thinking thoughts like, ‘Look at me scheduling and honoring my commitments!’ ‘Look at me doing hard things while everyone is asleep…’

And then one morning I don’t have these thoughts anymore. One morning I skip, next day I do it and it’s actually harder than before. Next time I half-ass it and hate every minute of it. And by the fourth attempt, I hear this thought, ‘This was way out of the park for you, let’s just sit down, regroup, and rethink the whole plan.’

What I know today that I did not know all the years, is that my brain is giving me these thoughts because it is seeking relief from expending effort, something it is designed to preserve.
What I don’t understand is why on the first week, when it took the most power to start the regimen, my brain was all in? Nothing has changed in the program from week one to week two. Why did my thoughts change?