Even though the Guys have helpfully offered some synonyms for your own phrase no, there isn’t any idiom or particular phrase for that.
For a recognised idiom, the closest you could get would prolly be tomorrow is another day but only in contradistinction, if for instance today has been a bad day and particularly when today has been very bad.
(See https://www.google.co.uk/search?q=annie+tommorrow+is+anothe+day&rlz=1C5CHFA_enGB718GB718&oq=annie+tommorrow+is+anothe+day&aqs=chrome..69i57.5111j0j7&sourceid=chrome&ie=UTF-8 )
For a phrase, your own I am going to have a fulfilling day tomorrow is fine in and of itself. It needs no alteration.
Meanwhile please consider:
I am keeping an English diary to practice my writing skills/writing…, and I tried to end today’s entry saying to myself: "I will have a fulfilling day tomorrow.”
I wanted it to sound determined, but I am not sure if there is an idiom or phrase for that.