Okay several things wrong here. Let me try and break it down to you from another perspective.
1) I have a 16 year old daughter. WHO SHOULD NOT BE HAVING SEX! If she is doing so with an 18 year old. Someone is calling the cops. PERIOD. She is underage. PERIOD.
I started dating my hubby at 16, he was 18. We were not having sex at that age. It's inappropriate and yes illegal. That's on him, true, but seeing as you are his fiance and are involved with her, delinquency of a minor, totally a possible charge against you. Sounds harsh, meant to be. You are the adult, you should know better. PERIOD. As the parents of a 16 year old who is incredibly hormonal and sex being an issue, you bet your sweet booty I would call the cops. Hell, my kid knows that even if he is also underage I will call the cops, cause we've discussed this, she's not old enough, responsible enough, or has enough value in consequences of her own actions. They would both be prosecuted in that case and I would do it.
2) When you hang out you too are a child. ARE YOU SHITTING ME?? I get it, it's a mental illness. I get it. I had a friend who would regress and the fact that she thought there was nothing wrong with doing this and letting her kids take care of her was a big problem. I have a mental illness. One that makes it impossible to function at times and leaves me catatonic.
Part of having a mental illness is being responsible for your own well being. That means, for me, that I have to be aware, I have to let my hubby know, my bf know, friends. Kids are now old enough to know and deal with things like me being slower to do things and have a hard time being motivated. NEVER, NEVER, are they responsible for my mental illness. I can tell you, seriously, that saying to anyone, in law enforcement, in the judicial system, that you were a 14 year old at the time may do only one thing for you. It will put you in a mental hospital instead of jail right away.
YOU, as yourself, as the 23 year old woman you are, knows that this is inappropriate. SO DO SOMETHING ABOUT IT! It is not her job to help take care of you while you are a 14 year old. It is not her responsibility to say that it's inappropriate and leave the moment you change into this other personality. It is YOURS. Set your boundaries.
This is SO not about you feeling fulfilled. This is about being responsible and a mature adult, WHICH YOU ARE.
Poly doesn't mean you GET to have every relationship you want. It means you CAN have more than one relationship. There are still boundaries, there are still times that you are not healthy enough for a relationship, or a specific relationship, or that they are not. Sucks, but that's how it is, and when you have a serious mental illness it's even MORE important to have your own boundaries and are making sure you are taking appropriate actions.