How do you write resignation letter due to misbehavior?

dear sir,

i wish to resign within no time due to the misbehaved and harassment from my superior. i worked with this organization for 5 years and 8 months without a single complaint and warning letter. my past is clear. i worked hard for this organization. but i commit a mistake before a few days due to my illness, i got a warning letter for that. i accept my mistake, but the behaviour from my superior is not acceptable, being a human i did that and i already submit the reason. everything was fine after that incident, but my area supervisor and my division is harassing me. even the client is happy with me, i did promise with my client that i will never repeat my mistake in future and i will work hard but my management is not happy with me, that's why i cant continue my job under pressure and harassment.