Hi Lelarose
So sorry to hear that things are really tough. It honestly does get better although i know it probably seems hollow just saying that now.
Tbh I don't think you are unusual in feeling really upset at the thought of never having a girl. I'm feeling something similar at the moment as due to relationship issues I doubt that I will be trying for a baby again and the prospect of any baby, but especially a girl is very remote for me. It's been hard getting rid of the baby stuff and bottles and wondering if and how it can ever happen again.
The thing is I do so love my son now. He really is so lovely, especially as he has started to interact properly (still not by talking but slyly pointing to everything he wants) and being so naughty and cute that I want to smother him with kisses for being so cheeky when he is still not asleep and grizzling at 9pm, that when I ask him what on earth does he want and he grins at me and points downstairs (not a chance, son!).
I also understand about your reactions to your sister - my family are really crap at dealing with anything bad so i have to continually cheerlead and pretend nothing is wrong as they simply cannot deal with it. Unfortunately sometimes people are crap at dealing with things and it's really disappointing when you should be able to rely on them, but it is their problem, not yours.
I am so sorry to hear that your dp's mother has died too. Glad that his mother saw and knew your son but it is awful to go through especially at this time for both of you. 
Re your concerns about bf; at around the same time I had a similar fear that my milk was drying up (think I posted a thread) as my boobs seemed to be permanently empty and my ds seemed to be unsatisfied. I used to be able to express a 5 oz bottle in 10 mins on each boobn and it went to below 2 oz for both pretty much overnight.
I was advised that it is quite common to lose an ability to express (your body is just doing the supply and demand thing and getting better at it so you don't have more than you need), but there are other medical and herbal ways to increase supply. I found stress could affect my letdown so this may be what you are experiencing atm too.
Re increasing supply: herbally try fenugreek capsules (from H&B or similar). Medically you could ring your doctor and ask to be prescribed domperidone (anti-reflux drug) that can have a side effect of increasing milk supply. (I asked my GP and got it). If your doctor will not supply it (mine initially had never heard of the effect of boosting supply) you can actually buy it over the counter, but i think it goes under at different name. In any case it is fine to take while bf. I took both fenugreek and domperidone for a couple of weeks and stopped as it definately seemed to work and my boobs became slightly engorged again.
And at 19months still producing enough milk to satisfy my little piggy's craving for more milk than ever so don't think that this is necessarily the end.
I have occasionally yelled in frustration at my son too. I was so ashamed once that i asked a friend who was totally attach parenting if she had ever just cried to her baby to just shut up. Yup she had, many, many times, and every other mum I have asked has done the same at least once. You are the main care-giver and you would not be human if once in a while you gave into your frustration in times of stress. Babies will not suffer for an occasional scream of vexation - they scream, you scream meh. They just remember that when they want food, comfort or a cuddle you will be there and they are safe and snug in your arms.
I really sympathise with the migraines though as a fellow sufferer. It would be great for you to be able to get more sleep as that can stop it but I find that if you get a bad one you can get clusters for a while after. Keep on forgetting to go to the doctors as they are really infrequent but was told there is some prescription medication for migraine (don't know if compatible with bf though)
Finally hugs. You are not alone and you will get through this x