How to act sober after almost 18h drinking and just waking? ?

Whatever you do, do not drive. However, have a jacket potato with er now, drink lots of water and have a cold shower, remember to clean your teeth and chew some gum, spray plenty of deodorant on, you don't want to smell like a brewery. Try to relax by getting a friend to take you and take some tissue and pretend you've got a cold (that way you can hide behind your tissue), good luck.