Oh rose... Firstly, you are not failing as a mother: you are doing what we all do - muddling through and making it up as we go along. :) You didn't expect your daughter to be pregnant so young - it is not something you would ever have planned for - so of course you're finding it hard... You're having to work out what to do while you're already in the middle of the situation. 
You sound like you're doing fine. Actually, you sound like you're doing pretty well... So cut yourself some slack! 
I think there are actually three totally different things going on for you here:
- You are trying to deal with your daughter going through that stage of being an infuriating, inconsiderate and thoughtless teenager;
- You are needing to find a new 'balance' of responsibilities in your family as your daughter starts training/work and is much busier and more focussed outside the home than she has ever been before;
- You are trying to come to terms with the fact that she is pregnant and so far you are desperately worried about it.
Ordinarily, you'd get to deal with these separately, one after the other - as different stages in her teenage/young adult life. But because your daughter is pregnant so young and still living at home, you are having to deal with all three simultaneously.
It might help if you tried to separate out the three in your head a bit, because I think they need different tactics... :)
- This is absolutely normal. I have the same problems with my 17 son (who obviously isn't pregnant!) You can approach this just as any other parent of a teen would. On the practical side, I'd advise that you never go in her room again! What goes on in there is her responsibility. You will have some difficult moments - when you run out of forks and she runs out of clothes, for instance - but if you draw this line now, she will work out some basic housekeeping within a few months.
From an emotional point of view, her attitude is absolutely normal too. Horrible and annoying, but normal . All that huffing and puffing and taking you for granted is what teenagers do! I agree that detaching yourself is a good idea, but personally I have never managed it well. Instead, my coping strategy is to remember to do nice things for myself. The more stressed I get with my teen, the more difficult I find it to remember to do nice things, but the more important it is! You will cope a lot better if you can built in regular treats for you! :)
- This is a practical problem that lots of families face - more and more, as kids stay living at home longer. IMO, there is no 'right answer' - you have to find what works for you. 50 hours is a lonnnnnnng week (I couldn't work that myself) - and your daughter is both pregnant and young and new to work. I think you can actually be proud of how well she is dealing with work. :) You need to decide whether or not you want to cook for her and/or do other chores for her while she settles into new responsibilities. This is your choice. You need to find the balance that works for your family. If you are happy cooking for her every night, then do. If you want her to share the cooking, then talk to her and agree who does what, when.
Bear in mind that she almost certainly will not stick to agreements, simply because she is a teenager (see 1 above!). If/when she doesn't do what she said she would, it isn't a sign of your failure as a mother or her bad character (though I know it feels like that!) - it is just a difficulty you are inevitably dealing with because you are doing 'stages' 1 and 2 simultaneously. You'll get there in the end :)
- This is the thing that makes everything else feel so worrying and difficult. I have never had a pregnant daughter, so I don't know how I'd deal with it. But I imagine it's incredibly scary and very worrying and a bit exciting, all at the same time. You need to come to terms with being a grand-parent, just as your daughter needs to come to terms with being a mother. I suspect that you won't really be able to work out what you feel and what to do until the baby is actually born, and then lots of things will just fall into place. Meanwhile, you are bound to worry.
The difficult thing is that the worry about her pregnancy and her becoming a mother also sort of 'spills out' all over everything else... So, she comes home late and you worry she's going to be a slack mum... She doesn't bring dirty plates down from her room and you worry she'll forget to feed the baby 
This is natural, but not really helpful... Her normal natural teenage nonsense now isn't any indication how how she's going to be as a mother!
Honestly, really and truly rose, it sounds like both you and your daughter are already doing well in a difficult situation, so you will probably be alright. I know you probably won't believe me when I say you sound like you're doing well, but I mean it. :) I think you will both rise to the challenge splendidly when the baby arrives!