One of the most difficult things about writing is cutting (no, I don't actually cut myself with a pointy object to get rid of my inner pain. I mean cutting words, as in the removal of).
It's hard, but it happens. For multiple very good reasons:
One: sometimes you write something and it's completely clever and witty and the dialogue is AMAZING and you're so proud it came out of your brain you can't WAIT to share it with everyone you know. And then you realize that nothing changed throughout the entire scene, it was complete self-indulgent CRAP and has absolutely no place whatsoever in your novel.
Two: sometimes a scene will take a long time to ferment. You work and rework and rethink so that it's exactly to your liking. And then something drastic changes in your plot line and you have to cut this scene you worked on for DAYS.
Three: On the reverse side, there's those sections you wrote that you KNEW were awful when you were penning them (but for godsake I only have like 30 minutes a day in between work, kids, husband, etc so that 30 minutes isn't always my best). And later you get to go back and get rid of the crap when you think of something better.
Either way, it's frustrating. Especially when you go from having 20,000 words to 10,000 words.
But sometimes, you have to do it. You have to kill a few for the benefit of the many. You may feel like you are moving backwards, but really you are going forward because your work will benefit as a result of your new found knowledge.
Don't forget to save your trimmings, you never know when that self-indulgent crap will come in handy.