Most Helpful Guy
Yes you are immature, not your fault, you are still under 18. By using the word, "fiance" I suspect you two are engaged to be married? Why at such a young age? I think you two need time to grow a little more and experience life more as adults. Don't be in rush to get married. This may be why he is backing off.
Most Helpful Girl
If you really are under eighteen I would seriously consider talking to him and taking things slow not getting engaged. Have a sit down and talk to him about it all and see if you can change or compromise so everyone is happy. Seriously though, wait until you're at least twenty one or so to think about getting engaged, it's a big step. Ultimately though you can't make him come back to you if he's not feeling it.