How do you know when you have outgrown a friendship? I have a friendship that is really hurting me. - K
Oddly enough, you have answered your own question.
The quick answer is that you and I will want to keep a relationship as long as the benefits outweigh the costs (and those of you whoo-hooing that you caught me doing my economist lady thing, you are right). The tricky part of that equation however is the only person who can decide on what you value and what rewards you get from giving it your time, energy and attention -- which are some of the costs -- is you. So, no one else can determine this for you.
The best indicator of a relationship problem is the way you feel, which is why you answered your own question. If you feel worn, drained, angry, difficult or anything like that, the system needs either a re-balance or the connection has to be dropped. The problem that you are facing, which bothers many Americans, is that we are taught that Good Friends are There for Each Other! Or that loyal true friends hang in there through the bad and the good. None of these imply that people are supposed to damage themselves on behalf of the friendship or other person.
If one person has done something such as become healthier, develop new interests or changed what s/he wants in a friendship, then the friendship has to change along. If it doesn't, then people who hang on to the old one are going to feel badly on some level.
So, the answer is that you know a relationship has to change when it starts to continually evoke more negative feelings than positive over a period of time.