Dear girl, you sounded terribly stressed.
Firstly, your little daughter doesn't NEED any expensive presents, or even any presents at all this Christmas. Babies of that age are very happy with a big cardboard box and a load of kitchen utensils. She will not remember this Christmas anyway. I know you want everything to be perfect, but you are really tired and struggling and something has to give. She won't mind!! There may be a toy library near you where you can borrow toys, even big ones, for a week for a very small fee. Babies lose interest in individual toys very quickly anyway, so changing them every week would suit you both.
I am more worried about you. Remember that nobody feels that well at 7.5 months pregnant, and please try to rest when your little girl naps. You will feel a lot better and more positive and strong once you have got past the newborn stage with the little one. The age gap between your daughters will not be large, and it will be hard work at first, but the rewards of having sisters so close together in age will make it all worth while.
As others have said, do you have contact with your local homestart? If you do not have a support network around you, having access to ladies whose children are past this really labour intensive stage can help bring perspective.
Think of the positives: You are having your children while your body is at peak fitness, you will be a young and fit mum to them once you've bounced back from the birth, and you can train for a career without way feeling older than the students who've gone on straight from school. The important thing to remember is that your little girls will be a delight to you, however bad things seem may seem now. Hang in there, and do not be afraid to ask for help. Everybody needs help.