I wish I'd had a more casual approach to dating in my early 20s - I lightened up in my late 20s and had a great time, but honestly I wasted the years when I could have had the most fun and adventures on trying to make every bloke I dated 'the one' and treating absolute pigs like princes.
It took SO long for the idea that love doesn't have to be poe-faced and earnest, and that not everything has to be love, to sink in.
Basically, I wish I'd been able to put my 35 year old brain in my 21 year old head. I'd have been unstoppable - don't get me wrong, I'm pretty happy with myself now, but I'd like to have started earlier!