Resignation Due To Illness

I've been working at this company for almost a year and I got hired without even getting my TIN(don't have one yet that time), clearances(NBI, Police, barangay), and SSS but my salaries are already deducted. I was in probationary period for 6 months then regularized but the contract signing for the latter happened only weeks ago outside the office. My first months' salaries were fine until they cannot collect from the client due to my "slow" progress resulting to partial and delayed(taking months) salaries. They even told me to work at home after office hours and weekends without pay and it's still ongoing.

Last week, I've went to a clinic to get my blood tested(blood chemistry) resulting to high cholesterol, triglycerides and "alert" LVDL which I think I got from stress eating, too much caffeine, and sleep deprivation. I really want to leave this company and I'm planning to include my health condition in my resignation letter.

How can I make my resignation letter weighty that it will really be considered as irrevocable? Should I attach my blood chemistry result? How about medical certificate telling that I am unfit? Also, can they file a case against me for not finishing the project? I didn't sign anything saying that I must finish it and it's not even part of the contract. I'm still willing to render the 30 day period.

Guide me please. Thanks in advance.

No need to attach any med cert. Just put in there 'for health reasons' and thank them for the opportunity and render the 30-day period. You'd be ok with only those instances.