First of all, don't judge yourself or your relationship or the person you're in the relationship with to harshly.. many wonderful healthy relationships go through a
co-dependent phase - until they learn how to negotiate for their valid needs.
Many times we are so happy to have someone to love, someone who loves us - finally -
that we will do anything to make them happy and make the relationship work.

Unfortunately, this information wasn't taught in High School and yet we may know very complicated math equations we may never use.. (nothing against math)
it's just that I feel we need more life skills in our education in order to learn how to be a happy society - slowly schools are becoming hip to this important factor in education.

So here it is simplified:

If it hurts you to help someone,
      but you do it anyway.. (especially again and again)
it's because there's no way to negotiate with them
(or re-negotiate as the case may be - because your needs or abilities have changed)
and tell them that it doesn't feel good - for whatever reason.
Then you are presently acting out a "co-dependant" relationship.
(They should want you to be comfortable if they love you.)
Whether you can't negotiate for your valid needs because of:
(or renegotiate as the case may be - because your needs or abilities have changed)
emotional manipulations,
drug &/or alcohol abuse
violent threats or deeds
abuse on any level (including psychological)
even our own fears, blocks, & insecurities
then it is co-dependant behaviors & our life and love for ourselves and others
is begging us right now for an upgrade (transparenting for a better life)
of the situation. Not the person (even through that may happen) themselves.
The change needs to happen inside you..
and saying "no" to what's not working is the first step.
This is not "tough love"
(something that only compounds the problem
of feeling rejected and not really having support)
it is real love and should be done in a very supportive,
honest, and loving way.

See: The Art Of Disarming the Bully (within and around us.)