Changedid, my first reaction is to shout "Please don't do it", to yourself, to your family and to the baby, but I may find this issue too emotive to be rational.
I was adopted and really feel the hole in myself, the rejection and the isolation, but maybe it is just worse because my adoptive parents were not really up to the job, they were emotionless, detached, dismissive and allowed me to be abused without doing anything about it. They had plenty of money and gave me lots of opportunities but this is not enough without the emotional input a child needs. I have read that adopted children become insinctively resistant to bonding with another adult to protect themselves against being hurt again and perhaps me and my bro (not blood) were like this, but our parents were still crap.
But, like someone said, there are so many people desperate for children nowadays and few children up for adoption that your baby seems likely to go to a home where he/she is really wanted and given lots of emotional input. When I met my birthmother she found it hard to deal with the fact that I had not had the better life she had wanted for me.
If the baby is likely to be abused if she/he stays with you, or all the children are likely to be taken into care as you fear could happen if you can't cope than I can see that adoption looks like the better option. I really don't get the feeling you would hurt any of your children though! You sound really caring and are putting a lot of thought into this because you care about your children. You don't sound like you are abusing your existing children. I just don't think you have it in you to seriously damage any of your children and I believe you can do it (keep the baby), with lots of help which you deserve!
I really do think that giving away your baby will be harder for you emotionally than keeping him/her. I could be wrong and having an emotional overreaction because of my own situation, but this is how it was for my birthmother: giving away her baby hurt her so much that she shut down her emotions and became quite a cold person. She has friends but they are more like aquaintences, there isn't a deep bond. She won't let her DH be too affectionate or call her affectionate names. She was less expressive with her DS who she had later on than she feels she should have been.
She wouldn't allow herself to feel too much because she didn't want to get hurt as much as she had been hurt before. When she met me she wouldn't allow herself to feel too much in case I was taken away from her/took myself away from her again, so she was quite cold and behaved strangely which caused arguments between us. We can only get along if we have an email only relationship now.
She has lived with guilt all her life and feeling not good enough because of what she did. This drove her to be a workaholic for a long time, needing to progress more and more to prove she was good enough. She made herself ill working so hard. She had little time for her DH who she 'told' to go and work abroad for a while. They live back together now and are getting on well. She developed this superior attitude to people (self-agrandisement) as a defence I think to how crap she felt about herself for what she did (and because of how her own family had treated her). She shows off the whole time and doesn't listen to other people (although this could just be a personality trait).
She was 19 when she first gave birth and the father didn't want to stay with her. In your case you have got your DH and other children around you. I'm not saying this makes it easier if you have other things that are difficult eg mental health problems. But just to echo what some others said, I have a friend who was given away at birth because her birthmother thought she couldn't cope mentally, then a year later she had a baby and kept it and went on to have 3 children who she kept. The effect on my friend of being given away when they were kept is devastating.
She keeps saying "What is wrong with me?" She thinks people can see something defective in her and that nobody will want her. She has had problems in all her relationships. She doesn't make friends easily and when she does she expects them to drop her any minute and continually pesters them, asking if we still like her, worrying that every little thing means that we are cross with her, being really demanding because if she doesn't have contact every day she feels we will disappear. This is so wearing that a lot of people do leave her. She seeks extra marital activity and threatens what she has got because she can never get enough reassurance of being wanted so she can't resist it.
I hope you don't feel got at by me, but I am just describing RL examples of the effects of adoption on all the people involved (although I don't know how siblings feel when their sibling is given away). These examples may not be how it is for everyone, they are just the ones that I know about.
If you can make it clear to your doctors and social workers that you really need lots of help when you have the baby otherwise you are in danger of further mental health problems and the welfare of all your children could be affected and you got the help would this not be a good option? I agree with what someone else said, don't be too proud to get ALL the help you can get from EVERYONE for the sake of your children. If you are finding things hard DO NOT keep it to yourself, make sure you keep telling people until you get the help you need.
I don't think it will be easy keeping the baby either, I think it will be very hard, but I definitely think that after a hellish few years, when you really feel you have learnt to cope, you will feel better about yourself and everyone will feel better than if you give the baby away and feel the pain for your whole lives.