You don't want to rely on willpower, self-control, or "feeling motivated" to continue a new behavior. If that's your plan, you *will* fail.

Change your environment and build habits. Ideally, you don't want to have a choice about whether or not to do the new behavior. It should be easier to do the "right thing" than to fall back to the old behaviors.

Also, use accountability, if possible. Knowing that someone else will know when you fail is a good motivator.