Be kind to yourself. You're going through a rough patch right now and you're dealing with some difficult things as well as grief, and all you can do is the best that you can. I am sure your adoptive mother knew you were just trying to console her about her daughter and make her feel better, not lie to her or deceive her. All of us want to have hope to get through things, sometimes that's all we have to hold on to. Allow yourself the time to grieve and heal and do what you need to for yourself. (((hugs)))

You’re doing your best. You can’t always take on the pain of others, but you are doing your very best to offer that comfort.
Stop and remember: it’s hard today. It hurts like hell right now.
This WILL change, though. Time passes and situations change...the pain you feel will lessen.
Take care of yourself, as you care for others. Hugs.

What you said, when confronted with the cancer knowledge, is actually the Only thing you could/should have said, so let that go, now!
Every chance you get, take a moment to contemplate something beautiful outside yourself...a sunset, a lovely lake, a nice piece of music. Under this kind of stress, you Need to take every opportunity to get a moment of peace & beauty, and it doesn't have to cost anything. It will also give back a bit of that feeling of control you understandably have lost right now. Best wishes!

Please remember to be kind to yourself and remember that women tend to be raised with the expectation that we are always supposed to be caregivers...we give and we give. It is okay to ask for help when the stress gets out of control and we feel like we're running on empty. Nobody has unlimited sources of energy, compassion, or hours in the day. And when it comes to comforting somebody going through grief and loss, you said what you felt was the best thing at the time. Being there is the most important thing. I agree with other comments about writing down your feelings and struggles in a journal if you don't already do so. And then write out a plan of attack on your finances and your budget. It is step by step, day by day and you will see progress. Personal finance guides are valuable and can be found online...check out Pinterest for budget guides, for example. I found them very helpful for setting out goals.

One day at a time. One disaster at a time. You did what you did to comfort a suffering relative, your kindness shines through. Today concentrate on breathing, everything else will happen in time. My partner doesn't get paid during summer, we works for local universities doing clearing.
Look after yourself, if you need to talk I'm here.

You did the right think for your family, it will pay off. You just wait and see.

Wow, that's a lot to shoulder :/
"The only easy day was yesterday." -- a Navy Seal maxim
I repeat this to myself when I wake up after catching some sleep during stressful times.
Sometimes, I have to emotionally detach from myself, how I'm feeling, and go on robot mode just to get the basics done.
I don't have any military training/experience, but I find it helpful to apply some of their teachings and discipline during times I want to give up.

Most times, being present for those grieving is best. I like to give them the platform to speak if they choose.