I'm working on perspective in drawings and I can't seem to get it right

especially when drawing from life. Need some help. Here are some problems I'm facing:
1) For example I try to draw my room, how can I tell where is the horizon line? And each time I look up from my sketchpad the view seems to change.
2) When I draw cubes using perspective where I connect from vanishing points, some will turn out looking a bit off/more rectangular. How can I make sure of the size? Do I need to measure precisely?
3) Objects with different sizes are even more confusing, especially when the ones with few horizon lines. How come? I thought there should be 1 horizon line in a drawing, no?

* Any suggestion on perspective drawing exercise?