TK ur shoulder looking big dawg
All i wear are basics too. I grew out of petit standards a long time ago, and now Uniqlo in my size. Just need a shirt now that fits my body type. I do have a girlfriend believe it or not!
I've had this exact problem. The following is from personal experience. I don't know if it's all "correct", but it has worked for me.
You need to strengthen your core and be able to recruit it during the squat. Do heavy ab work (weighted planks, GHR sit-ups, hanging leg raises, etc). Practice on taking the biggest breath of air you possibly can and pull it all into your stomach. At the same time, flex your lower back as hard as you can. Hold the tension for the whole squat.
I find it easier to get the form right if I squat straight down instead of trying to sit back a lot. Low bar sit back does not work with poor flexibility and weak core. If you do more of a high bar style squat, you don't want to use as much hip drive. You want to be able to activate the glutes so they can push you up. You should feel the glutes flex at the bottom.
The reason I hyperextended on the way up was partly because of chest rising too fast, but also because I was losing tension. If you are able to hold that tension - belly full of air, lower back flexed and glutes active, during the whole squat, you pretty much can't hyperextend. It helps to keep the upper body more upright. Do mobility work to get your knees far out to the sides (external rotation) and sit more straight down and it should come by itself.
Stop back squatting for now. Try goblet squats first and see if you can do them properly. When you can, progress to front squats. This is how far I've come after a couple of months of trying to address this problem. I will continue with front squats until I can do them correctly and heavy. After that it's on to box squats and finally to back squats.
Also, I think the reason your upper back is rounding is because your not holding on to something, a bar for instance.
@ maplefuturist: I'd echo much of what is said here. Goblet squats --> front squats --> back squats will help you get your core strong enough to back squat without hyperextending. Obviously hard to tell too much from a series of still photos, but I will say your bottom position has a few things going on worth addressing.
1. Looks like you are hitting the bare minimum as far as depth goes. You should be looking to get your hip crease below your knees at the bottom. This may be a mobility issue, in which case you should be working like crazy on mobilizing your hips/hamstrings.
2. Given that you've got no weight on your back, your torso angle at the bottom looks a bit extreme to me. This may be part of what leads to you feeling like your torso comes up too fast on the way up. I think that a better bottom position for you would be hips lower and torso more vertical.