It's hard enough shopping for yourself, let alone someone else. Thing is, it's not just anyone else - she's the love of your life, who probably has some pretty particular tastes. What's more, you're shopping for her engagement ring - jewelry she'll wear every day with every outfit for the rest of her life.
No worries! By taking the time to understand her personal style - as well as the engagement ring styles that correspond to it (plus more practical considerations about her lifestyle) - you'll do just fine. In fact, more than fine. You'll do great!