Hi Rose. I just wanted to tell you my story. I got pregnant at 19 by accident, I was in what I thought was a stable relationship at the time (we actually split up 6 months ago) and my mum was very much thinking like you - she wanted to support me in any choice, but there was no way she wanted to raise a baby again. She loves DS, but she is not a "baby" person. She said very clearly quite early on that she had her own life and she would not be babysitting all the time etc. I accepted this because, like SirBoobAlot, I felt that the baby was MY responsibility and I didn't want to pass him off at every opportunity. It probably helped that I had already moved out of home and was living with my boyfriend, but we did have to claim some benefits etc after I stopped work.
Although how well this will go down is going to depend on your relationship with her - my mum had always taken this kind of line e.g. when I wanted lifts to places, she said that I was asking too much and we had a bit of an argument where I said I didn't know how often I was supposed to be allowed to ask, and then we sat down and decided a certain number, e.g. one lift a week and I had to get the bus the rest of the time, unless it was somewhere I couldn't get a bus to. She never picked me up at 11 or later if I'd been out drinking - that was up to me to organise.
I think the best way to go about it is just be there for her emotionally but try not to offer her any solutions unless she asks for advice. So let her know that you will be there if she ever has a problem she really can't see the answer to, but that you trust her to make her own decisions. So if she starts stressing about money then you could suggest looking at claiming benefits, or if she wanted to do her apprenticeship and was worried it was going to cost too much then you could offer her a place to live at home. But for almost all problems, she can deal with them herself, without your involvement - you might need to nudge her in the right direction, but there are systems out there etc which will support her and enable her to stand on her own two feet. It's hard as well but try not to offer solutions automatically, like if she is saying "I really want this £500 pram" resist the temptation to say "Just get one for £20 from the local paper, you don't need a fancy one!" unless she actually asks "Mum, I just don't know how we are going to afford everything, what shall we do?" then you could suggest second hand stuff, ebay, local paper, freecycle, etc.
Equally try not to tell her her expectations are unrealistic or that she doesn't know what she is talking about, because that is just going to make her feel got at and probably react in an even more immature way. If you let her go and make her own mistakes and figure things out together with her boyfriend, she'll grow up quickly, because she will have to. So when she is going off saying "We are going to get a really nice place to rent" just wait and let them look around and find out prices etc, and see for themselves if it's realistic. No landlord is going to rent to them if the rent is ALL of their income, so don't worry that they are going to get into something they really can't afford. (But think very carefully if you are asked to be a guarantor for rent etc)
It's going to be hard, nobody is disputing that, but it's not impossible - she will be ok. She can always pick up her training at a later date.