Excitement is a starter. Inspiration sets many things in motion. They both, complete nothing.
The year is fresh so are the resolutions, one of which might be 200WAD. However the energy with which the resolutions were made often won't see it through.
Sooner or later you'll be back in the reality of your everyday life. For majority the energy will wade, that's when commitment must take the baton from excitement.
The excitement that spurred the decision might have been spontaneous; inspired by a talk or a desire to improve.
Commitment, however is deliberate.
Commitment is owned, it can't be outsourced.
As you already know, there's no "one thing". You needed the excitement energy to make the decision now you need to commitment to see it through. Commitment is seen in how you turn that plan to black and white. Commitment is seen in how you ensure you don't forget about the decision. Commitment is seen in you going back to the why of your decision. Commitment is actually daily.
Someone said "Commitment means staying loyal to what you said you were going to do long after the mood you said it in has left you.”
As you go through the year and realities begin to set in, you might ask yourself "what was i thinking when i made this decision"
You were completely okay when you made the decision, you only need to stay through to your decision even if it means grinding your teeth through it.
That, my friend is commitment.