Perform in an amatuer play

Keep using your tape. Go over your lines in your head between rehearsals. Spend more time working on the lines. Work on your lines alone, just before you go to bed. Some studies show that sleep helps solidify learning.

Try to associate what you’re doing on stage (such as action, moving around, etc.) with your lines. That way you’ll have an extra point of reference.
For each scene, think about what the scene is about. That will help your lines make more sense to you. What is your character trying to accomplish in the scene? What are the other characters doing to help or hinder your character’s actions? That will help you put your lines in context - they aren’t just lines, you know. They’re coming from your interactions with the other characters in the play. What are those interactions supposed to be like.