That you're just as much a parent as any other?
It took me a very long time, probably for loads of reasons but mainly because my high anxiety meant I felt that I was always 'looking after' my DC and not 'parenting' them.
One day (at the beginning of the lockdown measures) I went into a pharmacy, and the lady behind the counter told me that I should go home and keep safe, and that her sister had just had a baby prematurely that was in SCBU, her sister had complications which meant she couldn't be with her baby in the hospital and she told me that her sister was crying a lot and saying 'she doesn't think she's a real mum'. The lady herself was upset because she couldn't find any premature sized nappies for her new niece.
I suddenly had tonnes of advice and support for her based on my own feelings of 'not being a real mum'. She still remembers me when I go in because she said I helped her and her sister a lot. I was honestly surprised, because up until that point I didn't think I had any business giving anybody advice about being a parent. Since then I've been proudly throwing about the words 'my son' or 'my daughter' with reckless abandon 
I'm honestly interested to hear anybody's experiences, even if you felt like a mum/dad as soon as you heard your baby crying the first time. Or if your DC are preteens and you still don't feel like a parent?