I'm thankful you realize that getting married is a very serious step, and you're wise to wait until you are sure God has led you to make this very important commitment.
Elsewhere in your letter, you mention that many of your friends got married when they were still very young and many of them are already divorced. In some cases, they probably were too immature to understand the responsibilities of marriage, or perhaps they didn't realize the difference between infatuation and true love. Love involves commitment and sacrifice as well as mutual attraction, and the younger we are the harder it often is for us to understand this.
Why do I mention this? One reason is because this may help your mother understand why you haven't wanted to rush into marriage. The Bible says, "A simple man believes anything, but a prudent man gives thought to his steps" (Proverbs 14:15). Remember: Your mother loves you and wants what's best for you. A gentle reminder from you that you hope to get married someday, but don't want to make a mistake, should help her realize that she doesn't need to keep bringing this up.
The most important step you can take, however, is to commit your life to Christ, and to seek His will for your future. Don't be satisfied with anything (or anyone) less than God's will for your marriage. Give your life to Christ today.