I'm losing confidence with public speaking

I attend public speaking classes here, and I'm a few grades in. I've competed a few times. Won.

But lately, whenever I atrend these classes weekly, my speech preperation has been subpar. This plays with my speech delivery hence I feel less confident each time I have to present in the class.

We're given like 30 minutes to prepare a speech given a topic. I can't do impromptu and I'm never ready. I'm losing confidence. Maybe public speaking isn't for me.. Any advice?