You are being massively unreasonable.
Dealing with a person who experiences psychosis, psychotic episodes, or any other mental health problem is absolutely frustrating, worrying, and frightening at times. You are not unreasonable to think something to yourself along the lines of 'FFS, why can't you just take your medication, which clearly makes you better than you are now' - that's the frustation.
You are extremely unreasonable however to genuinely not realise the clue is in the name, mental illness. Meaning their mind is not functioning well. With episodes of psychosis, the very definition is losing touch with reality - and in this, not taking medication is an extremely common sign of such.
A person with an unmanaged health condition (and it is unmanaged, if they want to stop taking medication altogether for a severe mental health issue) is often not capable of making sound decisions that a person with a 'sound mind' is, and it's unreasonable to hold them to the same bar of mentally well people, as I say the clue is in the name really, mental health illness.
People can also go through swings of understanding they are unwell, and particularly with psychotic episodes, suddenly thinking that they are not unwell at all. Often accompanying this, is delusions and paranoia, very often including medication and feeling of people 'plotting against them'.
No medication is a cure. It relieves some of the symptoms and does improve matters, but that person still has a mental health illness which is prone to relapses, and relapses are extremely common. Very often people get used to a medication, and over time it reduces it effectiveness. This is another common symptom, when relapses occur (even small ones) when taking a dose of medication that previously helped them manage quite well. Often then the medication needs upped, or changed.
I agree that it is incredibly frustrating and upsetting. I agree that the impact of this on those closest to the sufferer is horrendous, stressful and often too much to bear. I also believe that often people suffering this type of thing need intervention, rather than relying on family and friends who will quickly succumb to the stress. There needs to be more support.
If the person has been on the books for years and has a CPN, then they are pretty fucking poorly OP. Families need far more support. However, you need to educate yourself and have a better understanding. I understand where your thoughts come from though, it is so upsetting.