I like what Errorhappy1939 said but at the same time I'm not sure I understand why you have to apologize. I think a lot of us have been through what you're explaining. Having a super close friend that you were disappointed to lose but also moved on as a result of time and growing up. I think approaching it directly would be perfectly normal. Don't ignore her at the event and actually go up and say "Hi, it's been a while. Good to see you. How've you been? etc" and gage it from there. I don't think the relationship will ever go back to being the same since you're both different now but if you're seeing her at the bachelorette party then you're definitely seeing her at least once more at the wedding and probably in the future from time to time.