If you don't mind me asking, can you tell me some of the things that lead up to that finality? Actions or behaviors?
Mum of three got there first, and I believe that she's right.
All I'll add is that whatever irresponsibility you are trying to save someone from, you will never be thanked, they'll just become sneakier and more contemptious of you.
You on the other hand are likely to completely loose touch with yourself and your own feelings and needs, you are likely to stop looking after yourself, you'll probably gain a ton of weight, you'll exhaust yourself, and become bitter, manipulative, passive aggressive and thoroughly toxic to be with...
Just look at the partners of chronic alcaholics, workaholics etc. and what they eventually become. It is very easy to sympathise with the irresponsible one; "sure, who wouldn't turn to drink with a partner like that?"
There are groups, for example Al Anon, who are formed by co dependants to heal from what they've fallen into. There are also good published books and I'm guessing some good forums like this one for co-dependants.
please, look after the one person who you do have control over - yourself