Oh Dear...Some unkind, unhelpful responses there!
You're not being unreasonable. You are stressed. And you'd like some support!
I can understand your situation. Without planning, or consultation, your DSD Is acting in an entitled way; to stay at your place whenever it suits, because it's more convenient than her mother's. You feel used, disregarded, that it affects you and you werent asked, and your DSD is old enough to have the autonomy of an adult, and yet is so naive she is acting like a child forgetting her manners. She should behave more responsibly. She could have gone about this better: should have asked you both, negotiated a flexible arrangemt. Offered keep perhaps (fat chance?) Or offered to cook, or babysit, or pointed out how grateful she is being under your roof.
Back in the real world, a mixed up 19 year old (who's mum might have disregarded her job need when she moved out of town) might be just being self centred and oblivious and assuming, largely because her choices are limited, and she will take whatever she can get and not be very generously hearted about it. She wants her bread buttered both sides and to not have to act courteously.
Here's the rub. You're gonna have to shake yourself down, hold your chin up, and make the best of this without arguing with D P. Take the annoyances of having a young adult in the house, and be the giving, sweet, step mum you want to be.
But you are perfectly able to negotiate a better arrangement. DSD can text you when she's on her way home perhaps and whether she is staying one night or two. Have a sit down 'meeting'. You can tell her that its lovely to have her around more but now she is working it also is changing the dynamic of the home, having an extra adult. Under other circumstances, she could be asked to pay rent - even for mid week only. Your home is not an entitlement now she's not little, it's a privelege, and if she wants an adult independent lifestyle then she shouldn't need reminding that things can get scratchy if she is treating the place like a hotel. She should reconsider her attitude to the place being an entitlement. And you can rule set (gently), to make sharing your home easier.
Also, however, perhaps you can dig deep, be really truly giving, and strike a new warmer relationship with the teen DSD. Acts of kindness to her (when she might well know she is an inconvenience ) will really help her to be more communicative and kinder to you.
Don't underestimate how moving it can be if you make a big gesture, or big gift. It can make a huge difference. Perhaps you could check when she's next coming and do a big family dinner to celebrate her new job? Really worth considering....it would warm DP'S heart too.