You can't change deep seated habits for another. Your love is fresh now, but marriage goes long in the future, with many disputes, and changes. If you're not wanting to truly explore vegetarianism for personal reasons, and she can't accept you for your diet, I don't see much hope.
You say you feel she's the one, enough to consider a diet change, but you're not exploring the diet change? That doesn't sit well with me. If you really loved her, I would think you'd do more than just question if you should.
I'd say you both need to really resolve this before even discussing marriage.
Don't let the perfect be the enemy of the good