it sounds like your ex just wants some time to explore....you've been her bf for most of her life..i guess she just wants to see if there's something she's been missing. who knows, if you two really are very serious (and it seems like you're quite close) she'll come back to you one day. like u said, u arent quite ready for marriage...i think she needs time before marriage to figure out what she really wants.
i have a similar prob with my ex...we've known each other since 6th grade and are know seniors in high school. we were togehter for about 2 years..in between we broke up for 3 months but got back together. the problem was that we saw eachother all the time, and were in 4 classes together and had the same group of friends. we broke up for good a month ago, and now he has a new gf. granted, we are a lot younger than you and your ex, but it was still difficult to have to see him everyday, smiling and happy with that other girl, me not being able to talk to him as much. it was really, really hard, and very painful for me. but i realized, tho, that since we've known each other so long it isn't worth it to get so angry after a break up. our friendship is really valuable, and i dont want to let jealously ruin 6 years of good times.
but hey, don't beat yourself up over this. i would say to let her have her space, get used to her new bf. but dont break off contact with her completely. lovelostlady is rite, she just needs her time. you'd do best to respect your ex's wishes for now, and not use pet words etc.
don't worry. it'll all turn out okay.