evedawn - I'm so sorry to hear that the situation has ground you down so far. Do tell your manager what the problem is, he will see it on your sick note or a self-cert form anyway. You are clinically diagnosed with depression so he can't say that you are making it up.
Years ago I'd have agreed with sir.alec but not now - mental health still frightens some people but employers have to be able to manage it, and it's certainly not a sign of weakness, so STOP thinking of yourself in that way. "Not coping" is not the same as "not being able to work when you are 100% overloaded and doing excess hours". We are outside your situation, remember, and we can see it for what it is.
Can I suggest that you contact your Trust's Occupational Health team, and ask for an appointment? If you refer yourself (which you can), then the outcome of the meeting is not shared with your manager (or shouldn't be!) - it's only if they refer you for an opinion that OH has to send them a report. That really might help, they will see trends developing in employee absence patterns.
I've been where you are, many years ago, and there is a solution, you just mght not be able to see it yet. You need to go with the flow for a few days, take any medication you've been prescribed, not stress about what people will be thinking of you. Your health comes first. In one instance I was able to leave the job which was distressing me, and your manager will have to report the diagnosis for your absence to the HR department as I have to for my team.
Keep talking - I know what you are feeling ♥