I volunteered in social care for the local council at a drop in centre for adults with MH issues for about 18 months. There was no training or supervision or any other sort of support, but it worked out great. I ran a craft group for 2hrs/week (plus prep time at home and buying supplies), and later on I started to do 1:1 outreach work (again, no training). I was hospitalised for a week in the middle of it and they were great letting me take 2 weeks off. I could've had more if I wanted, but I didn't need it. The only issue was that I had a panic attack before each shift for the entire 18 months, but I was able to push through it and get myself there each week. I stopped volunteering there because I moved away to complete a Uni course. I've actually struggled way more with uni (and had to leave) than I ever did volunteering. I never found that I was triggered by what other people were going through, but I do get triggered in IP environments, so I know that I couldn't volunteer there.
Wishing you all the best! It's normal to have doubts, but that doesn't mean it won't be a success
*Willow*