Yes, mum guilt is a real thing, and if it wasn't this you'd feel guilty about something else. This is part of being a mum. Also feelings of doubt and uncertainty after adopting are massively common, regardless of age. Post-adoption depression is also a thing too.
I adopted a 12-month old when I was forty. In my adoption group I was one of the younger ones, and I am by no means the oldest mum at school pick-up. Women are having children later. A friend of mine birthed her second child when she was 48. Your age is not as unusual as you think, particularly when you look more at adopters than birth mums.
The guilt is not helping you here. You need to put your energy into providing a safe base for your child. Even a child that age will pick up on your feelings of uncertainty.
It is what it is. You are not going to be able to change your age, so crack on with being the best parent you can be, and try to take yourself back to all the reasons you chose to do this. Toddlers are full on in terms of needing your energy, but as they get older, you don't need to be keeping up with them the whole time.
Build your support network and create the village your child needs with you at the heart of it. Whether that is friends, family, neighbours, nursery, childminder, school mums, sports clubs, church - when you come to realise you don't need to do this alone, it all becomes much easier. Good luck!