Hi Frank Gallagher, welcome
Glad you could find us here -- this is a group that understands the challenges you're having.
Wow, three people, and you're experiencing being in the role of defender most of the time -- either defending your GF or your daughter. That's exhausting and draining at best.
Have you ever heard of a "drama triangle"?
"Triangles" can happen in relationships where a third person is added on to a group of two, and changes the dynamic. Triangles aren't inherently always good or always bad. One example of a "positive" triangle is that for some couples, having a baby stabilizes the relationship (note, not saying this always happens, but it can, and that would be a positive example). Of course, a "negative" triangle could be when a couple has relationship issues, and a mutual friend steps in to "make it better", but ends up siding with one or the other of the couple. That destabilizes the couple's relationship.
"Drama triangles" are when there is a conflict between two people, and a third person steps in, with a negative or unhealthy outcome. There are usually set roles, though it's not like people verbalize "I want to be the defender this time!".
You're experiencing a pretty typical drama triangle. Let's say your GF is mad at your daughter, and yells at her "You never pick up your stuff! I've told you over and over, and you never listen!" Or something like that. Your GF is in the "persecutor" role, where the tone is "I am the one who is correct".
Then, let's say your D responds: "You're always yelling at me, and I didn't do anything wrong! Stop picking on me!" Your D is in the "victim" role, where the tone is "It isn't my fault".
Then, like, you said, you step in -- this time, to defend your D: "GF, just lay off of D. She's had a really hard day with remote schooling, just give her a break". You are in the "rescuer" role, where the tone is "I'm the one protecting people".
Frank Gallagher, does any of that resonate with you?
As you share that these conflicts between your GF and D almost inevitably turn into a fight, and you aren't sure what to do... please know, change can happen! It can turn out differently. It might take learning new and different skills on your part, but if you're up for that, we're here to share what we know. I know you love your D and your GF, and want things to be healthier. Shall we talk through some new stuff, and you can see how things go?
Cheers;
kells76