Well, I took & passed my driving test in '82 at 17, my HGV3 in '86 at 21, used to tow trailers in the army, so qualified for LGV2 when they changed the rules. Not sure I'd pass if had to take all the tests now, but spent several years incident free as an HGV driver, and 30K business miles p.a for the last 15 years with no problems.
Have never has specific trailer training, but consider I am safe when towing a braked trailer - to be fair with £30k of boat on the back I'm a defensive & cautious driver when trailing in any case!
Driving with a trailer is not technically difficult. You need to go slower - how many people do I see towing well above 60mph
?- leave more room, and anticipate better then when not towing. Not rocket science.
Manoevering & reversing is practice, & I bet we've all had good days when it's been perfect & days when we've had to take a couple of shunts to get it right. There are 2 one key principles - take it steady, & if it's not going right, straighten up & start again.