Highly under confident about myself

I feel really under confident about myself when I'm speaking to anyone or presenting anything or if I have to speak in a very large group of people online. I fumble in important calls, or speaking anything. Mostly i get lost while speaking or get anxious or fear when I'm in the spot. I totally get blank and start speaking gibberish or present things without any structure. This is severely impacting my career progression and also making me feel under confident day by day. My manager always bullies me and makes me feel bad about it all the time. He always questions my credibility and makes me feel like I'm dumb, even though I work really hard. I just don't know what to do. I even faced this in my last job. I just don't know how to help myself.