My eldest has had mental health problems since she was about 6. Shes nearly 14 now.
The saying about putting your oxygen mask on before helping others is really true. Ive tried various things over the years so here are some of my thoughts, suggestions.
Your child saying bedtime is when their mind is loud could be because for children its really the only time when they are still and quiet enough for their brain to be loud. So maybe try and create a still quiet time for them during the day to recreate this, the other thing is that if a child is in bed, and something tricky comes up, they can close their eyes. Its always easier to talk about something if youre in darkness and can close your eyes. Personally we've had great succes recreating this environment by going for a drive, they are still, not busy, their mind can be loud, they dont have to look at you, you can give them a blanket. Obviously you can only do one child at a time. I found it works best if we're doing a boring errand.
Children have a very limited way of explaining what it is they are actually feeling. There are some big emotions category and often they will think they are feeling sad for example, so you give advice for the emotion they name, but it could be the wrong emotion.
Model behaviour and strategies for them. Children don't often like being told what to do or to try something to help them feel better. But by naming your own feelings and your own strategies it actually helps them. For example I was feeling anxious the other night and for me, wobbling on a balance board with music really helps. My eldest came in to the kitchen and asked what I was doing so I explained how I was feeling, what helps me. Obviously in appropriate ways. It also gently shifts them into thinking about them finding out what works for them and taking a bit of ownership over their feelings and responses.
We talk a lot about the sensation in her body when she has a strong feeling, where is it, in their chest/belly is it fast, slow. Bodily sensations act as the body's first alarm system and getting them to listen to their body at the first signs means they can hopefully learn to catch it before the cycle spirals.
I have suffered with mental health issues from the age of 8, and the last 2 years ive learnt some great things, I can take photos of some of the resources ive found especially helpful for me and my daughter if you'd like.
It does get very draining and frustrating, but right now you have to meet them where they are at, even when the answer seems simple to you. Do you know anyone else who has children with MH difficulties as ive found that other parents in the same situation have been a great support. Just knowing you aren't alone and that other people are struggling with their children with mental health issues did help me.
Dont feel guilty, you're only human. Let me know if youd like those resources or anything, youre doing great!