Whether it's too big, too small, too old-fashioned, too trendy or too something else, if the ring isn't you, it's going to be hard to wear it for the rest of your life. So what do you do? First, relax, and then read on.
Wear it for a week.
Remember that time you got a pixie cut because you were obsessed with Carey Mulligan's? How you cried at first and thought you looked like awful? But then a few days later you realized you looked beautiful? Same goes for an engagement ring. Give it a chance, wear it for awhile and you may find out you really love it after all.
Think about what you dislike.
Are you disappointed with the size of the stone: It looks like a pebble and you were expecting a Kim Kardashian sized-rock. Are your complaints superficial or reasonable? If it's the former, remember most guys don't have Kanye's bank account and your fiancé probably bought what he can afford.
Be honest with your fiancé and then see if you can alter it.
Be sensitive to your guy's feelings. Tell him how much you love being engaged to him and wish you could love the ring too but it isn't quite your style. Don't make him feel like he screwed up. Mention something you like about it ("the diamond is beautiful!") so he knows he wasn't completely off base. Hopefully he'll put any hurt feelings aside and understand this ring just isn't working for you. If he bought a yellow gold ring, when you always wear silver, it may be possible to swap out a ring made of a metal you don't like for one that you do (a jeweler can use the same diamond).
Discuss getting an upgrade later.
Whether it's for a milestone anniversary or when your financial situation is flush, get a new ring or give the original one a makeover. That's something to look forward to!
Don't let the wrong ring overtake the fact that you're marrying the guy you're in love with. That's the most important part of being engaged; the ring is just a decoration—not the whole point of the proposal.