Most Helpful Girl
I wouldn't exactly say you're "justified" for feeling this way however, I do think it is a normal human emotion. When people dislike someone, they typically are "happy" I guess when things go wrong in that other person's life. Especially if that person hurt us at some point. So while yes, your "happiness" for her misfortune may be a little over the top, I think it's sort of understandable.
Although I must say, your fiance is with YOU. Obviously he does not care about this other girl so why are you stalking her FB page? I get that you don't like her and trust me, I completely know how you feel. I've been there, but you just need to let it go. Don't worry about her and what she does and what happens to her. Focus on yourself and your relationship. She may be younger and pretty but clearly your fiance finds you attractive and loves your other qualities too or he wouldn't be with you.