I have only just spotted this thread, but have read the whole thing and I just wanted to say that I think you are one of the bravest ladies I have EVER come across. I know you will think I am just saying that to be nice, but I am being totally honest. You're kids are going to be so proud of you when they get older, make sure you're around to feel their pride hun, because it will feel amazing.
You can do this, you already ARE doing this. You have been through a horrendous ordeal, you have been through so much horror that you must be finding it hard to cope. But it will get better, not straight away, but slowly it will get better; and sooner than you think you will realise that you don't feel so terrible.
As far as help on the bra goes: there are 2 bits of a bra size - the number and the letter - the number is the size of the band which goes around you, the letter is the size of your boob. So 32 is very small around you, 34 is a bit bigger, 36 is a bit bigger again. A is a smallish boob, B is a bit bigger, C is bigger again.
When you try a bra on: The first thing to do is loosen the shoulder straps equally (they always have them done up too tightly in shops). Then put it on and move your boobs around so they're comfortable. You should be able to get 2 fingers under the shoulder straps.
Firstly, getting the number right for the band which goes around you. If the band feels too tight around you, you need to try the next number up (so if a 34 feels too tight, try a 36). If when you look at the back the bra fastening bit is sliding up your back, rather than staying in a straight line with the front, you need to try a number down (so if a 36 slides up your back, try a 34).
Then for the boobs, if the bra material is creasing over your boobs or it looks like your boobs have too much room, you need to try a lower letter (so if a D is too roomy or creasing, try a C). Then turn sideways and look at the line of your boob where it meets your bra, there should be a smooth line, if it's pushing into your boob a bit so you have slight double-boob going on, you need a higher letter (so if a C gives you double-boob, try a D).
I know this might be too much info at the moment, I'm sorry if it's too much to take in, or I'm not making sense, but I have had an absolute nightmare with bras over the years (I'm a really weird size), and have found that this is the best method for me getting a good fit. Bras are sometimes confusing things and seeing as they always tell us that most women are wearing the wrong size there is no shame in being a bit clueless about them - I wore the wrong size for YEARS!!
I live near Cardiff, and if you ever fancy going bra shopping with me I would be very happy to go with you if you can get here, I'm not scared off by scars. 
Keep putting one foot in front of the other, you are doing SO well hun. Be kind to yourself right now, you have been through so much.