I don't think it was me that caused the issues. When we first got together I was looking forward to welcoming her and being a step mum
COMPLETELY the wrong attitude
A - you were the adult! Whatever the issues its not for a child to resolve them
B NOT for you to welcome her she was there as part of her fathers life BEFORE you. It was up to you to HOPE she would welcome you into her/their life.
C you do NOT immediately become 'stepmum' that relationship is not up to you, it develops over time with effort initially FROM you
My dds dad is a prize shit! Her stepmum (she's earned that title) was lovely from the beginning, she never expected anything from dd but gave her time, love and effort to first becoming a friend to dd, never undermined me or her dad in terms of discipline or boundaries, listened, calmly dealt with times when dd was not at her best as all children can be, NEVER blamed her for any issues within her relationship with dds dad.
He's army and lives other end of country, back when he wasn't quite such a shit dad he would visit alone as he had more leave than stepmum, or dd would go down to visit a few days early and he'd take extra leave so he and dd had time alone, stepmum NEVER had an issue with this.
When he's been overseas for months stepmum has even had dd visit just go see her half siblings and her. She has no obligation or duty to do any of this.
Sounds like you took the worst approach from the beginning