How can I convince my mom to support me in my marriage?

Just show her how responsible you are and how mature you are. Now that you're going to be married, you'll be financially independent and won't be relying on her for anything other than emotional support. Just show her how you'll be paying for your own college and room and board, car, etc. and she'll start to realize that you really have grown up and are ready to be making these kinds of decisions.