[Honestly, she just wants to scream instead of words, not because she has nothing to say but because she knows she'll mess it up if she talks. Unfortunately, the event isn't gonna let that stand, so now she's talking and yelling. Romantic!]

Undyne! I've had a crush on you since the day we met! But I thought you'd never accept me so I lied about stuff like-- l-like anime is just cartoons, it's not history! I'm just a nerd who doesn't deserve someone as cool and amazing as you but for some reason you like me anyway, and, and I love you s-so much!

[She's just got her hands pressed to her face now, blush having entirely overtaken it, trying to stop herself but utterly failing.]

You've forgiven me for every bad thing I've done and you still want to spend time with me and I didn't know how to tell you when you didn't remember because I thought you might not love me and I was t-too scared!!!