Telling a mum to stop worrying is like telling a fish to swim out of water 
But try not to swim your head with too many at once. Deal with one thing at a time.
Finances: do yourself a income and outgoings chart. Do an honest one right down to how many coffees you buy at a takeaway (thats my problem 
) and when its all down on paper see where you can cut or reduce something that is not necessary. ie mobile phone contract, sky packages, eating out (rare occurance) then go into the nitty gritty of fuel bills are there better companies with better offers on at the moment. It will help you see more clearly what you have or havent got when its laid out infront of you and you will feel you are doing something positive :)
DLA form is big and takes a while (says she who months on is still mulling her way through it) Its also a tough one on emotions so do it a few pages at a time Cerebra have a great guide in how to complete one. Again you can pat yourself on the back as you are getting things in motion.
As to your sons developement why dont you do your own little charts like an IEP for home? and monitor his progress on that. Set yourself your own little targets for him. Dont make them too difficult for you or for him. Stick it on the fridge and praise yourself and him big time when he achieves something :)
Then look at childcare, ABA etc The possiblility of childcare benefit maybe? I know nothing of ABA or its cost but I have read there are some charities that you could apply to for some help I think. Maybe someone may come along with more info on that.
Hope your day gets easier :)