When I was in HS, I joined debate club, where we spoke for 5-10 minutes at a time in front of anywhere from 5-20 club friends or opponents. When you do this repeatedly, for most people, your anxiety and nerves eventually go away, and it's just "what you do" now.
The same was true of musical performance: I hated performing until I did it about 50 times. Then I got comfortable and my average performance was no big deal.
Another idea: which language are you native in, and which are you using to speak confidently? If it's not your primary, that could be part of it. There's also focus-related disorders, if you have ADHD or similar, then your symptoms would make sense.
My Pet Peeve: Some people will interrupt others conversationally, and it's not nice. Conversation should be back/forth, not interrupting. You can always say "Let me finish my thought first". You can also have default responses to buy time: "Thank you, that's a great question" gives them a nice compliment and buys you 5 seconds to think.
And questions are a GOOD thing: they force you to really know the pitfalls, the details, and the trade-offs of your engineering strategy. People aren't trying to "getcha" and one-up you, they're trying to improve your approach, that's great for you if you can handle it emotionally :)
Good luck, you'll do well!!