A drama coach once said to us, "Don't just say the words - TELL it to me." It is the character's first time saying this (not yours) and it has to be communicated to the hearers.

Have you tried a gabble rehearsal? It's great for ensuring the words are in your brain! You say the whole monologue as fast as you can - no punctuation, pauses or anything! One looong run-on sentence!

Also - take each sentence and emphasize a different word each time you say it; you'll say it the number of times there are words in the sentence.

EG "The dog is black."
1. THE dog is black.
2. The DOG is black.
3. The dog IS black.
4. The dog is BLACK.

This last one will really help you with the first suggestion, and take the monologue beyond just memorizing, and making it your own.

In other words - play with the monologue and have fun with it.

Good luck!