I got u

(I love pranks. I always have and my girlfriend has always known this. We go back and forth pranking each other ALOT and we always go right in the nuts for April Fools Day. It's a fun tradition for us and we've done it the four years we've been together.
Like the title says, I pranked her with a fake proposal. I didn't do anything public but I just took a ring box and wrote APRIL FOOLS! around the inside of the box around where the ring would've been. I did make sure to set the scene like it'd be really romantic, with flowers and candles, stuff like that.
Well, she didn't take it well. At all. She actually cried (and at first I thought that meant my prank was a success) but I realized that she was actually really, really, hurt. I told her I was sorry it was a prank gone wrong and I'll never do it again and that just seemed to make her angrier. She's been at her mom's house since then and won't answer my calls and barely returns my texts.
I called her again frustrated to ask her what was going on with our relationship and she said that she didn't know, she needs more time, and that I'm an asshole for doing what I did, how she knows I'll never propose, and how her biological clock is ticking and I'm throwing that all in her face with my prank.
That's not what I wanted to do at all. I just wanted to prank her. I'm still not ready to get married so proposing to her to say sorry is out of the question but how do I tell her I'm sorry and get her to forgive me?
I'm really starting to stress out. I don't want to lose her. Advice on how to make it up to her is needed and welcome.
tl;dr: I pranked my GF with a fake proposal on april fools day. She still isn't talking to me. She's now rethinking our relationship and It's stressing me out.)