They sound like leeches, sucking the life blood out of you.
"Whereas another girl has been my friend for about 2 years and she is a genuinely good friend. I have chosen her to be my maid of honour ..." Great, focus on people you like.
"...and another of the girls said to her that so and so will be offended that I've not asked her." YOU decide who is your maid of honour, not your random frenemies.
"I can't believe they are trying to pressure me into even who I pick." Just resist the pressure to do what they want, it won't be in your best interests.
"They are jealous of this new friendship but I don't know why. I'm a njce person - why can't they understand other people see that in me and want to be my friend too?" They are jealous because they sound incredibly cruel and petty.
"I don't know why I'm scared to delete them. I think I'm scared they'll tell people I'm in the wrong but I need to stop being so bothered and think of myself, I know. " Do you know how you remove a blood sucking leach? "Carefully". Just drift away from them, so that no one need say you were mean, or had a big bust up, or said this or that etc. I mean they may say it, or they may not, but you do not need to tell them what utter shit friends they have been.
Just be slowly unavailable, busy elsewhere etc.
If they do not read what you write on line, they will not be too concerned about your on line life and if they cannot be arsed to come to your wedding, graciously accept their refusals. No arguments or drama. 'So sorry you cannot make it..." And then move on, smile inwardly as you know that eventually they will be out of your life.
Agree with Watto's approach.... "I decided just to stop contacting them and wait until they contacted me. After all, they are my friends and will wonder why I've not been in touch won't they? Well, 18 years on, I'm still waiting for them to get in touch. Getting shot of them was the best thing I ever did."
Also agree with TheLegendOfBeans, "Just drop them mentally and the rest will follow." Excellent wording.