I think sex in a new relationship is always very intense in the beginning, but once the feelings of lust start to calm down and you start getting to know each other properly, then sex tends to become less frequent.
Your relationship is possibly changing and progressing, where you are becoming more intimate on a mental and emotional level. You say she is an amazing girl and there are obviously many qualities you like about her, which I would think, make for a great longterm relationship. If it's just all about sex and nothing else, then it probably wouldn't last.
I can understand how your comments about it being the same and boring would hurt her deeply. She probably feels you are no longer interested in her. Maybe you just need to experiment more sexually, try different things and vocalise what you both like and want. Sex with the same partner doesn't have to be the same or boring.