What do you know now that you wish you'd known back then?

Advice from the future is hard to come by, but if you could do it all over again, what do you know now that you would like to tell your past self?

Whether it's something that you've learned over the years or just a useful fact that's only come to light recently, share with us in the comments the tidbits of wisdom or knowledge that you wish you had somehow been able to tell yourself in the past.