I don't know a lot about TBI's so i hope I'm not being insensitive on this issue or ignorant. I also don't know much about 18 year olds other than I used to be one (my kid is only 4.)

Seems she is not feeling in control of her life. Things changed that she didn't want. she will have to make these decisions on her own to gain control over what happened to her. You may need to just let her fail awhile and not push to fix it. See how that plays out. She may turn around. However, wth TBI she will need some help with that by presenting options (let her pick which one to pursue) to her and support - much support, when she is ready.

I can recall being in a situation without a TBI -but a situation where I felt trapped. One day I realized there are unlimited options in this world. (I think after reading the book called the option process or something like that.) You can choose just about any option in life. There are so many. You control your life, not the other way around. Even with disability this applies. You just have different tools to use to get to where you want to go. Maybe she doesn't see her options clearly. Find out what she wants and then ask her how she will get there. With that you can support her thought process to make it happen.

Best wishes to you.