How do I make friends as a middle aged woman?!

How the heck do we make friends as middle aged women? I’m 40. I wish I had just 1 friend! I’ve been talking to another school mum every now and then. She’s pretty cool. I really would love to be proper friends with her but I don’t know how to make it happen? (Omg, I know it sounds stupid). Do I ask her out for coffee or what? I’m afraid of rejection and she won’t want to be friends with me. She knows a tonne of people and I’m pretty sure she has a tonne of friends already! I think that is why I have no friends now, because I’m a total introvert and fear people don’t like me. Ugh adulting is so hard!