Multiple problems on the squat

[Eisen]

So I tried squatting again 126 kg.
I could only do it for 3, then I felt like I had to burp or vomit and I couldn't finish the fourth rep.
So I went to 120 kg but I could only manage three reps.
I think they're valid (i.e. I hit depth, but please correct me if not) but they're three instead of five and it's 6 kg less than what I had planned.
I did five sets of three because I couldn't finish a single set.
What happens is that I have to burp/vomit, I lose tension at the top of the squat and the bar feels like it's going to crush me.
I feel stupid writing this but that's it... I think I'm doing the Valsalva wrong: somehow im "eating" air but I'm not sure how.
Do I just have to suck it up? Can I really vomit while under the bar?
Also, I'm really upright. I'm not doing it on purpose... I think this is because I tried setting my back in extension.
Plus, my hips shoot back.
I tried fixing my grip and keeping the elbows down and now they really hurt (also my right shoulder). I'm sorry to bother you: I swear I'd love to get live coaching but, as far as I know, there are no SS coaches here in Italy.
My squat is a mess and it's a shame because it's the lift that fascinates me the most.

Video starts at 0:25
Shared album - Giorgio Capocasa - Google Photos

[Jeff Illingworth]

You take a very long time to set up without actually doing anything. Get yourself set and very tight under the bar before you stand up with it. This includes getting the middle of your feet under the bar and your hips back (just like in the squat, you follow the Master Cue). Squat the bar up and then stand there long enough to say your first name (in your mind, not out loud). Let the bar settle and then step back. Squeeze your shoulders back, brace/tighten everything up, and get your valsalva. When you take your valsalva breath, take a deep breath like you're getting ready to dive deep under water and it's the last air you'll get for awhile. When you do this, your stomach will probably end up sticking out some - let it. Don't try to suck your stomach in. After this, follow all the other cues from your fist thread.

Let your knees travel a little more forward. Your shins are still too vertical. Be sure, though, that they don't move anymore by the time you're halfway down. Getting a pair of weightlifting shoes will help with this.

For your grip, you're still letting your wrists roll under the bar. You really shouldn't be holding the bar up, you should be pinning it to your back.

Work on applying all of these corrections from your very first warm-up. Film every set you do and watch it between sets so you can see and fix problems as you go along.

[Eisen]

Thank you immensely for your valuable time,
I did a few sets trying to extend my back, moving the knees forward and not keeping the bar in the hands.
This looks a lot better to me but maybe it's because is only 135 lbs (It was after deadlifting and I didn't know how much I should put on the bar.) If it's good enough, I'll try doing my reps like this.
Also, I think I only get this weird feeling when I wear a belt... Could it be that I'm wearing it too high? Do you have any experience in merit?

This is the video: Shared album - Giorgio Capocasa - Google Photos

[Jeff Illingworth]

These are not really better. Also, don't stop at the bottom. Between now and your next training sessions, watch some good squat videos (like the teaching set on the Starting Strength YouTube channel, John Pettrizio's Instagram, Dr. Baraki's instagram, etc.). I think seeing good squats will help you.

Your belt may or may not be too high, try it lower. It looks like a lousy belt, though.

[Eisen]

I will. Thank you again.