Due to laziness, depression, and overall senioritis, I've let my grades fall to Cs/Ds and an F. (The F is in AP Physics, in which I got a C 1st semester, and I applied for engineering...it's absolutely ridiculous how far I've fallen behind in the material and I can't really see myself passing.) There's a lot of time left in the semester but I honestly don't know if I can get my grades up beyond mostly Cs and maybe a few Bs. I'm scared out of my mind because I don't have any real excuse or extenuating circumstances to tell colleges. Not sure what to do when I have no work ethic, motivation, or problem-solving skills to get myself out of this rut.
I've talked to my counselor a few times but she is very busy and teachers don't have all the time in the world to help me - maybe one or two will give opportunities for extra credit if I ask really nicely. I haven't been making any effort to contact my online tutor for physics because I haven't done the work in the first place. Essentially, there's nothing anyone can do if I don't want to help myself.
What are my chances of being rescinded by, for example, UCSD for mostly Cs second semester of my senior year?
According to an admissions expert interviewed in for this article, “A college’s acceptance is conditional on completing the senior year at the same level of performance as when the student was admitted." The article also references a study that found that "final grades are the reason for revoking admission 68.7 percent of the time."
If colleges see a dramatic drop in your overall grades, which seems to be your situation, your acceptance could be rescinded. However, I don't know the significance of the drop in your grades. If you were earning mostly As and your grades have dropped to mostly Cs, your application status is probably in danger. If you were a B student when you were accepted, it still might be a problem, but it's not as concerning. The F in Physics is undoubtedly a big problem no matter what, so you need to work on addressing that first!
If you there's nothing you can do to raise your grades to a level close to your past performance, I would recommend contacting the admissions office and letting them know about the problem. It's better to be upfront about the situation sooner rather than later. Try to contact an academic advisor from the college where you were accepted to see what you can do to preserve your admission. Here's the contact information for academic advising offices at each college within UCSD.
You also say that you have no excuse or extenuating circumstances that have caused this to happen, but I have to believe that something is going on to make you lose motivation so quickly. If you think you're depressed (which it sounds like might be the case), I'd encourage you to seek out therapy. It will be tough to start college in your current mental state even if your grades don't affect your admissions offer.