We really can’t know why you don’t have any friends. We don’t know you, haven’t seen you interact with others or really know anything but the short amount of info you gave us.
You have a fiancé? Have you tried meeting friends through his friends? I think you should discuss this with your fiancé, and try to find the money to find a therapist. It sounds like therapy would be good for you.
It does sound like your jealousy of your former friends 15 years ago probably put people off, but that’s just going off what you wrote.
Ask your sister to invite you and introduce you to people.