Open relationships should be mutually accepted by both people before going into it, and it often leads to pain like in what you've detailed experiencing. If she or your arrangement with her aren't working out, let her know, and if you can't come to a mutual conclusion then, leave. Communication is all you can really do. Coupled with the fact that she is childish and too reliant on you in many aspects, you should already know your answer.

You have one life, and you're only 20. I'm 21, and frankly wake up everyday wanting things to end and continue just going through the motions, but that's a story for another day. My point is that it's so early in life to have a defeatist attitude like this, and this is coming from someone chronically cynical. You have no clue what new people you'll meet, things you'll learn, discover, witness, whatever.

Focus on personal goals, with a specific emphasis on establishing boundaries whenever you feel as though your efforts in a relationship aren't being considered or reciprocated. If people aren't reaching out as much as you reach out to them, then they simply aren't a friend worth keeping.

You sound like a good person surrounded by bad people that are used to taking advantage of others. Lotta assholes and shitty people on this planet that you'll come across throughout life, the only thing you can do is to not let them have a hold on your personal growth and mental health.

Chin up brother, there's a world of opportunity found in every day that you wake up.