I think my marriage of 20+ years probably wouldn't have lasted if not for the birth of my daughter
That does not sound very good. How would you describe the current state of your marriage?
I can see that we've grown apart and that we're not the same people who we used to be when we got married
It can be okay to want to get a divorce if you are both unhappy in your relationship, but you should consider your daughter before you make any decisions.  If she is not yet old enough to handle that kind of disruption, it might be better for you and your wife to stay together, and raise her as a couple.
My daughter is 23 now and we're still married but I don't feel like I am married.
Then I would consider a divorce since your daughter is an adult and she will be able to handle it. You need to look out for yourself and be happy. You deserve that, and staying in this marriage will just make you feel like a zombie.