Regarding getting help and trying harder, usually people do make as much effort as they possibly can to get better. However, there is no sure fix. Sometimes there is no medication which helps enough (in treatment resistant illness). And while long term therapy may help somewhat (if the sufferer can afford anything more than what little the NHS can give), it doesn't always make much difference.
Yes, of course it's OK to have boundaries. You can only do what you can do, within reason. However I think people can be a bit too quick to think sufferers are lazy, don't want to recover, etc.Unfortunately recovery can seem an impossible goal, because a symptom of some mental illnesses is 'knowing' everything is dreadfully wrong and can never be made right.
No-one wants to be having horrific thoughts, to lose all their friends, to feel their life is worth nothing, to lose their job, to be told they aren't trying hard enough. It is degrading, unbelievably painful and BTW it can affect the nicest, kindest and most wonderful people.
Illness doesn't 'make them selfish', they aren't deliberately putting themselves before you. They are terrorised by their illness and it takes over their thoughts. If someone had a broken arm, would it be 'selfish' of them to not play tennis with you?
I understand that people have their own lives to lead but that is enough. There isn't any need to try to justify that with thoughtless comments about people with mental health difficulties being supposedly selfish and lazy.