I don't know that I would consider your behaviour as codependent necessarily, if you were married, your problems would be his problems, etc.

I would step back a little though, for my own stress level; you have to make sure you are healthy and doing okay before helping others or you won't be able to help -- remember the airplane oxygen mantra about putting on your own mask before you help children and aged/others having difficulty. But how to step back effectively?

My husband does our financial investment work and, since I am not involved in the day-to-day part of it it is a bit confusing and intimidating to me. I'm an anxious sort in the first place so the combination of my natural tendencies and the scope of our investments makes things worse. I asked my husband to write me a "book" on what to do if he died/got seriously ill so I had to take over the finances. Could you maybe think about and write out your analysis of your boyfriend's financial situation and difficulties and offer something in writing to him to help him? You could word it in a primarily educational manner to help him with the things you are expert at that he is aware you are expert at and relies on you to help with. Then you could give some overall pitfalls that many people have (choosing the wrong advisers/people around him :-) and make some suggestions of what you would do/have done?

It might make you feel a bit better as the two of you would have something tangible to refer to instead of the worries just wandering around his head in a mess he can't make sense of and you stressing about his stressing. Instead you could say, "oh, I hope that's on page 12" and point it out to him and you could feel a bit better knowing you were helping without having to be quite so intensely involved?

For yourself, I would look at his situation and see how you would solve it it were your situation and if it is solvable at all or if it is just going to be a mess for ever and ever or until the business goes under and he's left with nothing, etc. If it is just painful to watch but his struggles will eventually get him to dry land I'd keep that in mind as a sort of hope but if you think he's going to end up drowning then I would either try to step in more forcefully and find someone he would/could trust to tell him that and direct him or I would decide to step away as you don't want to go down with him?