Self cert for a week, contact your doctor for a follow up fit for work, and have that stating you are to only work your contracted hours for the next two weeks, or have them sign you off for two weeks..on your return, in your welcome back, state your reason as to your agreement with your manager was not honoured, and you felt pressurised into working extra hours, which put you back in your mental health issues.
A good manager will not "forget" a support plan he has put in place with you, and he will honour that agreement, despite pressure from senior team to get the overtime filled. A bad manager will not "forget" the support plan either, but will ignore it and pressurise you into working overtime, as you are easily manipulated and he knows it!
If you're not a union member, I would urge you to become one, and have them sit in with you at every meeting, they will also ask to see the support plan put in place, and query why it was not adhered to?? The support plan is a written agreement between you and your manager in supporting your attendance, by breaking that agreement, the manager has contributed to your further absence, and a good rep will push for that absence to be disregarded in any future absence %.
You could, instead, have a colleague sit in, but that colleague would need to be of a strong personality with a lot of knowledge on procedures, and available for every meeting, also be trusted enough not to gossip! If the colleague is not available for a meeting, they can suggest another colleague sit in, whereas if you're a union member, they must adjourn until your chosen rep is available, but if not available for a length of time, another union rep will sit in, and that rep will have all the relevant information, having been made aware of the circumstances beforehand.
I certainly would not put a letter in any managers pigeonhole, stating your mental health issues, which is then left for any one entering that office, to read! Anything relating to your medical history must be kept in your personnel file, as it's protected under the data protection act, choosing to leave it unsecured in a pigeonhole voids that protection. The onus is not on you to remind a manager of your support plan, and the reasons why it was put in place

DO NOT RESIGN!! If you resign, then you forfeit any benefits you may be entitled to for 6 months.