How to be honest about having depression

[trekking1]

I have let down the charity I volunteer at, not showing up and not letting them know. It is because I am going through a depressive episode.

Can someone please help me draft a message telling them why I haven't shown up and saying that I do want to keep volunteering but this might happen again because my MH is so bad right now.

When I try to draft the message I get sick to my stomach with shame and I just can't do it

How about something like this OP?
Dear X
I'm sorry that I didn't attend Y today/yesterday/this week. At the moment I am experiencing depression and so I don't feel able to commit to volunteering as I usually because my priority needs to be my health. I'd still like to continue working with you when my health improves. I hope it will ok to contact you again on Z date with an update on my availability.
Yours etc

I hope you feel better soon.