Memorization is a skill that is developed by practice. Although some people may have naturally good memories, others develop their ability to memorize materials by do so regularly; the more you practice any skill, the more adept you become. If you have little practice in memorization, it will take more time for you to memorize a text than it would take someone who was more practiced in the skill.
When you are working on memorizing the speech, eliminate all distractions. Turn your phone, music, computer, and television off and work from a hard copy of the speech.
There are two main components to successful memorization. The first is reading the speech aloud multiple times. You remember far more of what you say than what you read.
The second technique involves breaking down the task of memorizing. Read the first sentence of the speech aloud several times. Next, without looking at your text, say the first sentence aloud from memory several times. Check the written version of the speech to make sure you have memorized the sentence correctly. Next memorize the second sentence using a similar process. Then repeat the first and second sentences together several times until you are secure in your knowledge of both of them. Then add in a third sentence and repeat the process until you have learned the entire speech.