I’m the same way, for some reason I can say what I want to say but as soon as I hit that record button my brain goes haywire. I’d say just try different things, that initial syllable after you hit record is gonna be the hardest. If you can trim the video, hit record, take a deep breath, maybe say some other words before Taking to try to get a rhythm going with your speech, then you can cut that beginning part out. I’ve had to record myself for my French pronunciation practice, video interviews, etc. Never have I gotten a good attempt on the first couple tries. It’s frustrating and it takes time, but you will have a good take I promise you.