I know it is difficult if you don't come from a tactile family. I'm the only child and although I know my mum loves me to bits and would die in my place if given a choice, she finds it hard, and doesn't do touchy-feely.
However, they are your friends! Tell them just what you have told us and ask them to help you get over it, explaining that you will feel awkward at first and may even flinch away from their touch, but you will get there eventually even if you always find you have to make an effort. Good luck.