I think this can be an especial challenge for older children (she came to you aged 8 from what you have written?) with clearer memories of 'before you'.
Before DD's birth mother died I had to remind myself sometimes that we were not in competition (it helped that she was supportive of my role as 'other mother') - most children have two parents and love them both and have a different relationship with them both.
After all, 2/3 of her life happened before you became 'Mum'. I have read - and it fits with our experience - that becoming a family comes in cycles and with traditions. Every day you are there, you make sure she is safe, has enough food, is comforted if she needs it, has clean clothes etc. Every birthday, Xmas, other special occasions becomes a time when you, as a family 'always' does XYZ. Gradually this will become a bigger and bigger percentage of her life.
Every developmental step means working through it all again, with a slightly different perspective, with different triggers (one friend, adopted by her step-father, only felt the loss of her birth father when pregnant herself, and being asked questions about family medical issues.
Puberty is a really tough one - DD2 has had regular respite with me since aged 2, and hardly any contact with her birth mother since then, and was adopted by me at 5. She recently had a few days of intense unhappiness and missing birth mother, for the first time ever. I am sure hormones are part of that, but also reaching a more mature understanding of what she has lost (especially as her first mum died unexpectedly a few years ago, she has no choice or chance to get to know her or ask questions in the future).
Try not to be too upset or worried, although that is easier to say than to follow! It sounds as if you do have a trusting connection with her, and she has the emotional skills to unpick and articulate her feelings. From what you have written you are already her 'safe base' - that will increase with time. Your joint memories and traditions will increase with time.