Advice on stubborn 18 mth old feeding herself!?

I know you can't make an omelette without breaking a few eggs but my girl is driving me crazy! Up until now we've been lucky she is an easy baby to feed and loves her food. The past few days she has decided that unless she can feed herself with a fork and spoon she's not gonna eat. OK we say - she has to learn and to experiment, then we see the mess - food everywhere constantly, in her hair all over her trousers, the highchair, the floor, the walls. Every meal time is chaos and the highchair, floor everything has to be thoroughly washed. We've even begun feeding her in her nappy and bib to minimise clothes washing. She gets a spoonful of food and then brings the spoon up vertically so of course it all falls off! I've tried loading the spoon up for her and helping her guide the spoon but she screams till I let her do it herself. I'm getting stressed. Any tips? Advice? Thanks!