Hi Samantha. I'm a student as well and so I know how frustrating this can be! I'm sure it's different for everyone but here are a couple things that have been helping me this semester so far.
1. Does your school offer any sort of disability services program? I was hesitant at first to get registered, and just forced myself through last semester like any normal student, but I've finally gotten in touch with them and I'm really glad I did. For me they offer lots of options for anxiety-ridden students (time-and-a-half for test-taking, alternate testing room, other students volunteering to take notes for you if you can't focus, etc) as well as a few things that are specifically helpful for my PTSD - for instance, I can occasionally ask for a grace period of one day for assignments if I am unexpectedly struggling with flashbacks, etc. the night before and just can't get it done. I haven't used any of these yet, and I'm sure I won't use them often if at all, but it is REALLY nice to know I have those options in case things get really bad.
2. Change of scenery. This might just be me. I tend to isolate in my room or the library when I study and eventually I just zone out and can't do it anymore (and want to go to sleep, lol). A trip to a coffee shop to do homework or even just taking a walk while reading can be massively helpful for me. I find that when I change my environment frequently it wakes up my brain and redirects it.
3. Journaling. If your thoughts are swirling and you can't get them out, try just writing them out and getting them out of there.
4. Cleaning, exercise, self-care. My mood and focus tends to be much improved if my room is clean, my bed is made, I'm showered and ready to go. Also, doing some manual task like organizing, vacuuming, etc. for a study break keeps my body occupied and gives my mind a break.
5. Nutrition. Again, might just be me. I know I'm frequently dehydrated, the things I eat are not giving me energy, and the caffeine/sugar high from that coffee an hour ago is doing nothing to calm my shakes and my nerves.
As far as concentration while reading and such, I gotta say I'm in the same boat. :P You can try reading aloud, underlining/following your place with a pencil... That's a tough one.
Because you're a senior, your options might be more limited, but I am taking close to the minimum as far as units go and I am SO glad I dropped the extra units I had originally planned to take. I initially beat myself up about doing so, but I am really grateful I went with my gut. There is no way I could have handled another two classes' worth of work right now.
This post is a bit of a ramble, sorry - hope there's something in there that might help.