This is also another challenge you are probably dealing with that you don't realize:
It's important that you pay attention to how you try to "please" others. From what you have shared in your original post where you talked about everything you experienced and how hard you tried to help this individual, you were not appreciated and ended up broken down. And the anger you are feeling actually is justified in that your time and effort was not appreciated by this GF.
What you need to see when you sit and review this entire experience that turned out so badly, is to recognize the things you did that did work in this experience. You did experience "some" accomplishments in this effort, however, there were things you did and gave into with this individual that you also need to see that you now have to accept responsibility for. Now, my effort to say this to you is coming from my desire to not criticize you or contribute to you feeling like you failed or to contribute to your grief and pain. Instead, it's from one very hurt person to another. And in all honesty, I wish I had been exposed to a presence that helped me understand these dynamics discussed in these videos I have posted for you.
I tend to WANT to understand the why, and it's been that way my entire life. Lots of people respond with "forget about the why and JUST". Well, THANK GOD there are people who DO explore the why factor. It's because of these individuals that we are able to click on a video like the ones I have posted for you, that really help us see the forest through the trees. No one chooses to get caught up in the same dynamic only to once again get hurt.
Also, a child can grow up with a parent that is a narcissist and INSISTS that child become a codependent. It's no wonder so many struggle with depression and low self esteem issues or never seem to fill a void in themselves, often for the rest of their lives even.
If you think about your anger when it comes to the GF that basically chewed you up and spit you out and how you see other's latch onto her because she can sing? Well, all that means is she continues to have something that she can use to take advantage of others just as she did you. And she will most likely find another sap she can use and discard. Thing is, not every person who has "something" actually ends up creating a true positive from it. Sometimes the star they carry simply burns out and they are left with nothing too. And typically, if someone is a narcissist, they tend to leave a trail of broken people just like you too.
It's best to cut your losses and learn how to better identify the kind of person that will never appreciate you no matter what you do for them. It's better to pay attention to how you may be attracted to the very kind of person that you will never please, fix, change or get the respect you deserve from. It's also very important to identify this kind of person because they do know they can draw you back into their game of using you if they want to.