"To be really honest, I'm feeling really weighed down by the lack of positivity in our conversations. I need you to find someone who can actually support you with the issues you are dealing with. I can't only hear about the bad things".
OP....you need to realize this is who you are for them, someone to dump their shit on. Other people are getting other versions of them, the version that smiles and laughs and is helpful, and you are who they download garbage on. Don't be that person. Have a boundary. "Look, for every time you tell me something shitty about your life you are going to have to come up with a positive, too. I want to be your friend but this is not friendship".
It's going to end the 'friendship' because this is our role for them but...who gives a fuck. You'll be done.