This is my draft response:
According to employment law, you do not have to 'accept' my resignation.
I have repeatedly stated that I don't feel I am the right fit for the role for numerous reasons, and I have therefore tendered my resignation so that I am not in a position to disappoint either myself or you as my employer.
I am sympathetic to your predicament as an employer, having been in your situation myself, but what I must make clear, is that it is absolutely NOT ok to make contractual changes to an employee's terms of employment without mutual agreement.
I have stated that the job is affecting my mental health and my family dynamic - a problem I drew your attention to at the very beginning of our dialogue.
I do not agree with your proposed changes to my notice period. I have given 4 weeks' notice which is not only in keeping with acas regulations but is over and above statutory requirements.
One of the reasons I am feeling so stressed and overwhelmed is your unremitting inability to accept my decision.
I have given the role a three month trial and have come to the decision that it is not the right fit for me. Were the boot on the other foot, and, as per the contract I have been sent, you wanted to terminate my employment, would my pleas of 'but I don't have another job yet' be any more valid? No. I appreciate your efforts to keep me, and I'm appreciative of your faith in me but for any kind of relationship to work, BOTH parties must believe in it.
Therefore, as previously stated, my last day will be 25th July. I will continue to discharge my duties to the standard you have come to expect until then, upon which I will look forward to receiving due recompense for all hours and expenses charged.
Kind regards