OP, no one can advise you on your daughter's reaction. We don't know her. You do.
Unless she has a significant history of mental health issues, please don't put your life on hold. You will not be helping her, and she will only struggle more so the older she gets. She will also take this lesson into other parts of her life, which won't benefit her in her own relationships.
I would not move your partner in just yet unless you have been together for 3 or 4 years, and moving in has been an open conversation for you all, regardless of how your DD feels.
I would keep reassuring your daughter, I would spend one on one time with her; without your partner and then I would build up to a meeting, more, 'we are going for a walk on x day at x time', or an activity that you know your daughter will love. If you keep waiting for her to agree, it'll never happen. You have to help her to understand that life is all about change. Change is uncomfortable, but it leads to growth, and we have to try. You're not encouraging her to become a criminal. You're trying to help her move forward with you while supporting that transition from just the 2 of you to 3, maybe 4 of you. Life doesn't stop for anyone, not even our children. The difference is that we can be kind about how we proceed and at what pace we proceed.
Your daughter is responding to this situation purely from her limbic system. Read or listen to the Chimp Paradox. It will benefit your understanding of her feelings a huge amount.
All of the above is truly dependent on how long you've been in this relationship. Your daughters age matters. Is she 13, 15, or 17?
Please don't be afraid to live. You're more than 'Dad', and one day, your daughter will be more than just your little girl. I doubt that as an adult, she'd want to know she's responsible for a lonely old age for you, not to mention sole caring responsibilities for her. We all need support and opportunities for growth.
Help her to navigate this challenge. Don't hide from it or be frozen by it.
Oh, and my experience comes from being a long-time family law practitioner, working closely with Cafcass, Mediators, Child Psychologists, Judges, Social Services, Local Authorities, Teachers, Therapists and, being a mum to 3 daughters for almost 2 decades.
Also, my husband is 50. He absolutely could be a Dad at 50, and he'd be fantastic at it.