Help with appropriate boundaries

My 13 year old SD has always been in my life. Her dad and I were best friends before getting together and so I was asked to be SDs god mother (a story for another day). My SD doesn’t know a life without me. We have had 50/50 custody of SD from age 4 to 10 and have had 100% from 10 to now when her mom decided to pick up and move 2.5 hours away one summer day. SD and I have had a fantastic relationship up until this year. I was an assistant coach her sports team, signed her up and drove her to all her extra curriculars for many years and we’ve always just had a really great girl bond.

This changed recently when my husband and I were approached by HBM one night threatening to take us to court if she wasn’t given 100% custody come the new school year. Come to find out this is what SD wants and her and her mom have decided this was going to happen MONTHS ago. For context, SD had been asking us to sign her up for sports for next year, begged us to keep her in the same school next year while we were looking to buy a home and when we have checked in with her to see if she was happy with the time she had with her mom, her reply was always “I’m good, I’m not going anywhere”. Blind sided is an understatement!

My husband and I cried for days, even weeks. Ever since this day, SD has been incredibly mean, nasty and downright deceitful. We’ve caught her in many lies, and she’s turning into a completely different kid. One would even say that she is mirroring her mom. Her HBM is a narcissistic pathological liar who has stolen a lot from many people. I don’t recognize this kid at all and quite frankly, she’s so mean I don’t even want to be around her most days.

My question for everyone is - what are some appropriate boundaries I can put in place for the time being until she goes to her moms, without being a complete bitch? I want to still show her unconditional love and I want to enjoy the time we have together while we have it, but holy fuck she is just miserable. She says the meanest things about me to me, she doesn’t listen to me when I speak to her/just ignores me, everything she tells me is a lie and just makes me feel awful. I know teenage hood is hard and girls always go through this stage but I think that her mom has been telling her to treat me like this. I didn’t even get a Happy Mothers Day.

What can I do to survive the next four months without breaking down completely?