It sounds like her friends may be speaking out of turn. They may think they are trying to protect her but it's not their place to be discussing your relationship with you. I think you need to tell them to mind their own business next time it's brought up.
For now, though, you need to sit down with your GF and hear in her own words what she wants/needs before you beat yourself up over any of this. And using her age as an explanation is really just an excuse. I've known plenty of 20 year olds that are perfectly capable of handling a relationship and just as many 40 year olds who still aren't. The fact is she knew when you two started dating that you had a BF and thought she could handle it at the time. If that's no longer true, what's changed?
Also, have you discussed your feelings or needs with either of them? I'm seeing a lot about how what they feel and what they want but do they know your feelings and needs? Any relationship, poly or not, requires all parties to be aware of and responsive to each others feelings and needs for it to work.
Good luck what ever you decide to do. Being in love can take a lot out of you regardless of how many people are involved