I write to let you know i have decided to terminate my contract with the Club.My last day of work will be 12th November 2019.I have decided to come back to Poland and find a job that complies with my education and skills
W obydwu zdaniach jest zwrot i have decided i to troche nie pasuje jak to zmienic
...because I intend to return to Poland (change the point of view).
This is your reason for quitting. Stick to this line. Do not spill your guts unprovoked.
You are giving a letter of resignation. Be wise. You might need references from your present employer in the future. You never know.
Thank him for hiring you. Offer help in training your replacement. Wish the company success.