Thanks for writing back - really helpful to hear a little more. I will answer what i can and hopefully give you some pointers of where to go with this.
Firstly a CPN is a Community Psychiatric Nurse. They work in teams with other people like psychiatrists (like me but for adults), psychologists and social workers. They look after people who are not in hospital but who need a lot of support at home. I do not know what your Mum's diagnosis is so do not know what her signs of becoming unwell would be, but if she has a CPN, it probably means her illness is pretty disabling at times and is long term.
Her care plan, even if it is out of date, should mention things like 'relapse signs' and 'crisis plans' and you will be mentioned on it too, or you should be, both as a carer, but also as a 'dependent'.
It is possible, as you are good at putting a brave face on things to avoid standing out more than you do already, as you say, that people looking after your Mum have not realised you are not being looked after properly at times and that, because of your own difficulties, you cannot look after yourself when your Mum can't. They might be making assumptions based on your age and not realise you don't function at your 'age level' in all areas of life.
There are a few ways forward I think. One, as we talked about, is writing to your mum. But I understand your reluctance to do this, given what you say about not being confident you can read her state of mind.
You mention you have a young carers worker. And possibly other professionals involved in your care? You could ask one of your team to speak to your Mum's CPN on your behalf. This would not be breaking confidentiality if it is you giving them information about what it is like for you and that you need advice about how to help your Mum and how to read the signs of her becoming unwell. Carers often speak to the team looking after their family member, in fact it is encouraged as it helps the team, in this case, your Mum's CPN, know more about how she is doing. People with mental health problems often don't know they are unwell when they are, which is why information from people close to them, like carers, is so helpful. You should also be able to ask for a 'carer's assessment' which is a meeting for you with your Mum's team to go through your needs as a carer.
The other thing worth mentioning is that many areas of the country have 'crisis teams' which work all day every day to manage patients with mental health problems in crisis. You should be able to get their contact details. If you prefer on-line contact they are likely to have an email address you could write to. As will the CPN, I'm sure you could email him/her rather than speaking on the phone if you find that easier.
I hope that's given you some ideas. It is clear that you care for, and about, your Mum a great deal. But you will be able to function much better as a carer and as a person in your own right, if you ask for a bit more support.
all best wishes