I deliberately pointed out that becoming a parent did change my perspective.
You remember being 14 from being a child.
You're looking at the situation through a very different lens right now.
I'm sure her behaviour is challenging.
However, I promise you'll feel very differently about the behaviour of a 14 year old when you are the parent.
I'm sure you have tried. I've felt at times like the bloody little engine puffing up the hill "I hope I can, I think I can" until exhaustion.
I've learned that you have to keep chugging along. Keeping the "moral" high ground.
Most importantly is you and your DP need to be on the same page.
We have house rules and "parent" rules.
House rules are for everyone and any adult can enforce them (and the consequences are known). So doing chores for example, treating everyone in the house with respect. Consequences can be withdrawn devices, WiFi access, pocket money but they've all been communicated in advance.
If I enforce a house rule and consequence DH backs me up. Always.
House rules are all about how we live together and function as a blended family.
Parent rules are about issues such as ear piercing for example. Or educational choices.
I express my opinion to DH on the latter but respect that any decision is for him and his ex to make as parents.
Maybe it would help your relationship with your future DSD if you and DP clarified issues such as this?
I don't subscribe to the view that SM's shouldn't ever discipline or enforce boundaries in your home. Being a pushover isn't helpful. When you have your own kids it's bloody impossible if as an adult you have to accommodate a child who can do "whatever" unless "dad" is around to say "no" and it's shit even without kids.
However, you need to be clear about the extent of your authority and back each other up.
Maybe a little less kindness and a bit more fostering of agreed boundaries is required to foster some respect?
Just a thought....