Yup. Throw that decline on top of the ADD, and lately, "thinking" almost starts to feel like a misnomer.
SO much this. So smart, so much potential and expectation. BP first hit (with certainty) with a huge depression first semester of third year at university. (It hit out of nowhere -- I was having the very best time of my life before that -- which was probably hypomania actually). Had to drop out, couldn't get out of room for... at least a couple of months. There was no real help. I was made once to go see someone who asked if I was "sad" "because I didn't have a BF" or something. I was speechless. This was not "sad", this was incapacitating. BF? As if! Might as well ask if I'd like a little cry because they didn't have my favorite dessert in the cafeteria that day! So very very far off the mark. The only thing I DID know was that it wasn't "because" of anything. It.made.no.sense! I knew nothing about mental illness even though I grew up in the midst of it. I can guarantee the words were never uttered. From what I've read things have improved a lot. At least there is awareness now. If you think denial and ignorance is big now...you should've seen it then.
But I digress... came out of high school super smart (scored 99th percentile for college bound on ACTs) and entered university. Declared 4 wildly different majors in 2 years, dropped out the 3rd. Didn't stabilized enough till 10 years later and managed an AAS (2 yr degree), #1 in my program, another 10 years pass, started "go" number 3. Turned out suicidal for much of the first of...7(?) years at that school, changed program to a different 2 year program. Got a degree in neither. Later, the bottom dropped out. Followed by misdiagnosis, mistreatment, and then finally the beginnings of proper diagnosis. Should've happened 25 years earlier.
I refer to them as "College Round One!" "Round Two!" "Round Three!" and "Round Four!" Went to college enough to be a doctor. Got a 2 year degree.

All this to say... given a choice, I'd rather not have had it go that way. But there simply is not a choice in having BP or not. But that doesn't mean there aren't choices. I didn't have them out of sheer ignorance. Who knows? Maybe things would have gone differently if I'd known what was going on and gotten help for it. There is a LOT more help and awareness out there now. Have you spoken with a school counsellor? They can let you find out resources that are available to you. Talking with a therapist could be very helpful too. They can help you work with tools such as CBT (cognitive behavioral therapy) in which thought patterns are examined. The way we think can have a really big impact on how we experience things. If a pattern is not helpful, there are techniques to combat it (with a big ol' reality check) and help replace it with a productive one. Sometimes, this sort of work is just the ticket. Because sometimes, situational things and thought patterns are at the root of someone's depression. There are other things to think about too... like eating good food, getting proper sleep, exercise (doesn't have to be elaborate -- walking is good). I'm not saying, "Eat a salad, think happy thoughts and all will be well!" Lol. Just that these are some things that can either really help or really hurt us mentally, not just physically like people tend to think. It's also a good idea to go to the doctor to rule out potential "physical" causes. For instance, symptoms from thyroid problems can look just like depression.

These are good places to start. It may turn out they work for you, and that would be great! Or it might turn out that some medicine would be helpful. Not everyone who takes medicine for a mental condition has to do so forever. (And it's not either/or. People who take meds also deal with things like sleep patterns and food choices in trying to keep as well as possible.)

Oh! Jumped back in for an edit, forgetting to mention something important. Yeah, we've got some sucky stories(!) But it's not ALWAYS that way. We've had people on here who've done really well and accomplished a lot. Some straight through, others tackling it in pieces. How it goes depends on a lot of different things, but I just wanted to mention that.

p.s. I hadn't seen you mention doing any of the above kind of stuff. Forgive me if I assumed incorrectly.

p.p.s. Speaking of assuming...It's easy to assume people are shiny shiny happy happy just because it may look that way from the outside. Everyone's got insecurities and crap. Try not to let an illusion make you feel bad, ok?