Find something worth becoming excellent for.

Excellence pursued for its own sake tends to run out of fuel, because the standard is abstract and the effort is real. Attached to something you care about, the same effort becomes obvious, and the standard rises without you having to force it. The reason does the work that discipline was doing.

If your motivation is thin, the missing piece may not be more discipline but something worth the trouble. Find the thing you would want to be genuinely good at for someone else’s sake.