PANIC OVER!!!
Went back, had different shop assistant and a lot of help from some poor other guy that was trying to buy a bike, but instead ended up cycling up and down the road next to me for ages.
Tried the 56 again, was actually bigger than I remembered it during my anxiety attacks, then rode the 58, which actually felt quite comfortable - bit like a long cushy sofa, but I think that was mainly because the handlebars ended up at the same height with the saddle. It failed the "obscured hub" test tho and was admittedly too long, would have needed a shorter stem already, and the stock one is only 100.
So, hopped back on the 56, definitely think it's the right size now, just not used to having my handlebars lower than my bum, but it's all in proportion, the stem is the right length etc. So I stuck with the original 56 after all...., just hope it won't take too long to get used to the position and the 100 mm stem gives me plenty of options if I want to go longer...
Glad I went tho, feel much more reassured and won't have to spend all week stressing.
So, no further reassurance needed (but thanks again for trying), now you can all just go back to telling me how lovely my new bike is!!!! All compliments highly welcome!!!!