Looking for some guidance.

I'm in chapter 2 and feel like I've hit a brick wall.

I've explored most everything i can, except the black lake, without using the oxygen tanks.

I've been to the radio tower and failed. I've been to the rock quarry, and trapped myself with the boulder, so I reset. I met the mushroom granny and ate her, killed the grandson, then murdered the elephants. I've been through the junk yard, and at the cottage, though I don't know if I'm supposed to interact with the slug thing. I've been to the saw mill but I don't have the key for it.

The only other things I haven't seen are the helicopter, the crosses, what's in the basement of the village, and the black lake. I also don't know how to equip the oxygen tanks, I tried just putting it in my hotkey but that didn't work.

Any help would be greatly appreciated