Blimey Mum2neve, you're not alone! I too have a 3rd old daughter and I am 23 weeks pregnant. During my last pregnancy I suffered very badly from depression which affected my relationship with my husband and the way I viewed the baby growing inside me. Luckily once I had my dd, all the depression faded away (mind you I did get progesterone treatment!). So when I found out I was pregnant again, I felt just the same as you. I didn't want another one, I was worried in case the depression came back, I wondered how I would cope, etc.
The depression has come back, but I am managing to fight it with lots of help from Mumsnet, friends, the GP, the midwife and the Community Psychatric Nurses. I still have doubts about wanting this baby, but I know now that there is light at the end of the tunnel. I have been there before and come out the other end, so I will again, and so will you. Even mothers on here who have planned their second/third/fourth babies all feel that worry and doubt when they do find themselves pregnant. It's only natural.
As to how you will cope with sickness etc, well you just do! It's amazing what your body can put up with! Get lots of support from your partner and borrow some children's videos from the library. Now your child is 3, will they be starting nursery in Sept? If so, that will give you a little breathing space. Make sure you communicate with your partner, tell him how you are feeling and what your worries are, he will need to support you. Also have a word with your GP and midwife to see what advice and support they can offer you. Visit my website here to find out more about pre and post-natal depression and the treatments available for it.
You know how much love you have for your child, you will have that same amount of love for your new baby. Really you will. And your other child is old enough to be a bit of help to you, it's not like having a baby and a toddler to cope with.
I'm sure you will get loads of replies from Mumsnetters who have felt exactly the same as you! So relax a little, what you are experiencing is perfectly normal. And who says that things can't be different this time?