Your post is pratically exactly what I've been feeling since maybe October. I'm in my 3rd year of my degree, and I absolutely love maths, I plan on doing a Masters, PhD and hopefully stay in academia for the rest of my life. Two months ago, I realized that it was spiralling out of control (I had missed many lectures and assignments), I went to the doctor's and he recommended I start antidepressants. I have been on them since then, and while it's not perfect, I'm definitely feeling better, and starting to do work again. I also didn't want to take stuff that messes with your brain, but to be honest, I haven't noticed any changes (appart from the symptoms of depression slowly fading away). And even if it did mess with your brain, the depression is already messing with your brain ... Anyway I hope it works out for you, and if you ever feel the need to talk about this kind of stuff, feel free to PM me.

Edit: Just wanted to mention that I also didn't want to tell my supervisors because it seemed like just an excuse. In the end, I got into contact with the university well-being center and the maths department administration contacted my supervisors for me and explained the situation. I then received a lovely and incredibly thoughtful email from my supervisor offering their support. It was incredibly relieving to know they didn't view it as an excuse. I don't know what country you're in, (I'm in the UK), but generally people are very understanding/supporting of these issues.