@Bec1985 welcome to the forums
That sounds like a really hard situation for you - we're really glad you reached out for support.
Congratulations on your new relationship - it must be an exciting time for you, apart from the challenges. 
From your daughter's perspective, it sounds like it has just been you and her for a long time now - 5yrs is a large chunk of an 11yr old's life. Is it possible that she was really happy and content with how things were with just the two of you?
If so, it could be a grieving process for her where she feels like she's losing something precious (her exclusive relationship with you) - in contrast to your feelings of excitement at gaining a new relationship.
If that's the case, your daughter might need time to adjust.
Can I ask how long you've been seeing your new partner?
I'm not sure if it helps, but I've heard it can take around 5-7 years for many blended families to really blend.
I'm wondering whether it would be possible to go really slowly with your daughter for now - eg. catching up your partner without your daughter there, and keeping visits with her short, fun and light without too much expectation on how it should go?
I'm not sure if that helps - you know your daughter best and what might help her to process and adjust to this new situation in her life.
Are there things that have helped her deal with big changes or losses in her life before?
In case it's helpful, there's an article on our website about being a good step-parent that might have some tips for you and your partner to try.
