OK. I can sympathise fully when you say you find it hard to trust your feelings, when mental health is involved it is so hard for the sufferer to know when they are feeling 'true' feelings and when they are 'distorted' thoughts. But look at your actions then. You have been a non-couple for a long long time, living separately. To me it sounds like your true feelings are to say no more. I assume you have spoken to a therapist at some point over the 12 years about this too?
A big question is, why why why is he managing your finances?
Do you really feel you are useless? Do you really let other people do everything for you? I doubt it. Does your health situation make you feel so? Or is it your life situation? If he was not around, do you honestly believe you could not make it from one day to the next just fine?? Do you work? You are paying off a mortgage or I assume so. You can get to your savings, just walk into the branch and explain to the manager, and it will get sorted (or make a call depending on what kind of place you have it in). He should not have access to your money under these circumstances. To log in, walk into bank and ask someone to show you how. Tax return, ask a friend to help, or hire an accountant to show you? I assume you could even ask in a job centre for help there on how to do it, or some such organisation? Everyone has a first time to do things. Unless you are illiterate (obviously not as you are typing away here), or innumerate (I assume not), it is just a confidence thing. Logging onto a bank account is logistically no harder then logging onto mumsnet!
If I were that unconfident, from what you have said, you should worry whether you have just become too reliant on him. You can do it alone. Many people do. And you can ask for help to do so. A history of mental health issues does not mean you are not just as good as anyone else and as or more capable. And perhaps a change is exactly what you need to help yourself get better. For example, with depression (just as an example of something I know a bit about) the literature I have read strongly says that no matter how much support and help you get you risk descending back down into the illness unless you make changes. The illness is your body's way or telling you that you are not happy and doing things wrong, whether in the wrong job, needing to learn how to re-establish boundaries, needing to pace yourself differently, needing to look after yourself better and value yourself.
You live on your own, work, earn money, own a house, raise children. You just need to believe in you. None of what you say sounds like a useless person, you do sound like you need time away from this quasi-relationship, so you can live life. So if you want out, just tell him I want out, and don't listen to his protestations. If you have made up your mind, his opinions don't matter.