What should i do about a friend who is severely shy?

we always make plans to do things but then she'll back out bc shes not sure she wants to and i know its bc she is shy because i was once as shy as her but i'm trying to coax her out of it by saying she'll be fine but it gets kinda annoying n frustrating to me bc i want her not to be afraid to do things because she'll end up missing out on all the fun what should i say to put her mind at ease about even the simplest things in life?