Your last post made me chuckle a bit.
Of course it wouldn’t be the same if she were 57, ie 9 years older than you.
Let’s not pretend that her being 21 years younger than you - with a firm young body, etc - had nothing to do with what attracted you to her.
Also her being in a different phase of life - not as much baggage; not yet needing to have kids, etc.
It’s fine to admit it to yourself and the world.
Dating young people is fun - they aren’t weighed down by the pressures of life, are not as damaged or spoiled by life yet. Also - they are less set in their ways and their lives are more flexible - so they can adjust and fit into your life more easily than a partner closer to your age.
Finally - and that is the cliche bit, but also just the reality - you are smack in the age where men start feeling their age. And a much younger partner gives them that jolt of youth. There is a reason men say ‘you are as old as the woman you *k’...
So - no reason why you shouldn’t enjoy this relationship for as long as it last. You get a lot out of it. And eventually - if she sticks around long enough - you’ll get a young carer.
Turning to her - if she were my friend/daughter - I’d tell her not to waste much time with you. Unless you were quite well-off and then, of course, I’d say she should marry you pronto. (Joking, obviously).
Young women do get something from a relationship with older men. More mature men tend to be more secure, treat them better and take care of them in a way a younger man won’t. You are giving her stability and a place to live while she studies.
And as it happens - men in their 40s are still quite attractive and energetic - so 20years difference isn’t felt as much.
But after men hit mid 50s, the large age gap tends to start catching up with most couples. And eventually the couple ends up looking like ZetaJones/Douglas - where a still youthful woman is with a man looking like her father.
Your gf doesn’t realise these facts of life yet. She is too young for that. She has a perspective of a young person when it comes to relationships - where if it feels good right now - it’s great and meant to be. She also doesn’t yet seem to know if she wants children - as it seems not yet on her radar. As it isn’t for many women before they hit mid 30s and biological urge to procreate seems to hit us like a train.
Anyway - OP - you don’t need to overthink. Just enjoy what you have. It may or may not last - but it’s true of any relationship.
You are getting a lot out of this one - so count your blessings.
She is getting something too, but it’s more complicated as she is also giving up things she isn’t aware of. But essentially it’s her choice.