go to your local Pregnancy Resource Center! a lot of them help you with an adoption plan but also can help you financially if you choose to place for adoption
I'm so sorry you're dealing with all this. Feeling miserable right now sounds like a perfectly sane response to your overwhelm. Can you contact a women's centre or other free/low cost service for counselling asap? With practical help if you're in the UK, Homestart are an organisation supporting families who might want support for any reason. People are volunteers and often parents/grandparents themselves. Is your oldest on the waiting list for an assessment? Perhaps one of the assessors for that could suggest support related to their behaviour while you're waiting. Could they go to nursery a couple times a week if not already to give you a little break? About your thoughts on adoption, you don't have to decide anything right now or until after the baby is born. Maybe for now concentrate on you and your family and then it might help you see more clearly what decision you feel is best for your third. Please try to talk to a trusted friend too xx
Rather than adoption, is a termination not an option? Oh, and get on decent contraception!
You’re going through so much and I’m sorry you’re dealing with this. Only you know what’s best for you and your family, and I wish you the best for your health and recovery. Whatever you decide, have no regrets and be at peace with your decision.