To answer the question, for us mere mortals who start as adults, I think it's the work of a lifetime.
I don't know how often you play/practice , but I think 21 in 18 months is decent going. That's a similar level to a few of my group who've been playing for years.
Regarding playing other sports, this is a really tough game. Many sports, such as football, are relatively easy for some people to get fairly good at (even if the pros are on a different level). That isn't the case with Golf, where beginners luck pretty much doesn't exist. I've seen other folk who are generally good sportsmen join our group expecting to be able to be competitive , score 60+ on the short 9 hole we play, and never play again. I'm in a similar place to where you are, so I know the early stages can be frustrating.
Once your full swing becomes consistent, you'll just keep on improving.
Yes they should be allowed to go. My wife is a grammar school maths teacher and they let all pupils use the bathroom if they need to. Its usually once during the course of a lesson the school permits them to leave the class unless they have a medical exemption or are unwell.
Many (id hope all schools) allow the pupils to drink water during class to keep them hydrated so chances are they will need to spend a penny at some point. When i was at school it was breaktimes only and there were a few instances i remember, especially in year 7 (11 year olds), of a few kids who needed the loo and being denied then wetting themselves in class. That sort of humiliation stays with a child for some time.
Being able to use the toilet is a basic human right, plus trying to hold it in can lead to bladder problems and urinary infections.