I've almost erased through the paper. lol I'd scan it to show you what I mean but my scanner is a PoS and my drawing might now show up cause I've been drawing so lightly.
The trouble with oddly shaped and over-sized breasts is more than likely due to the angle you're drawing from. A side
(side boob LOL) view is simple, you won't screw up...but rotate the body 30 degrees and one will appear closer than the other, which isn't so simple anymore. If you're on such an angle, sketch a rough box for the upper body and another for the lower.
Don't erase your screw-ups. There's a type of blue pencil you can buy from hobby/art shops. I can't remember exactly what it's called, but it doesn't show up on the image when you use a scanner. They're great for rough drafts/skeletons. Although, using the blue pencil is somewhat a step ahead. It's mainly for a draft before ink and colour. I used to use India Ink and Photoshop for colour.
Like Enlenne said, start with a rough skeleton. Just like how she drew the head, I do the same to a degree. I also add lines through the diameter to give a proper proportion of the eyes/eyebrow, nose, lips, and jaw line. The same can be done with each part of the body.
Your best bet for proportions is to research the human anatomy. It really helped me a lot, especially with drawing hands. I'm under the assumption you know where to find some good anatomy anyway, lol.
In all honesty, proportion comes with practice. Hence, don't erase your screw ups. They'll show you where you went wrong and give less room for error.
I haven't drawn for years, but I use to be pretty obsessive about it. Everything from basic Mega-man sketches
(GREAT for a beginner), Spawn cover art, Tomb Raider <3 fight scenes, to twisted hallucinogenic inspired Alice in Wonderland murals.