Just when I think I'm doing great, it comes back

Sorry this might get long. 20 yr old male here. Have had severe depression and anxiety all my life. This semester at college went amazingly well so far. I have a therapist to talk to, but didn't even need to see her yet. I was exercising like crazy, lifting a couple days a week and running almost every day. I meditate and do yoga. I'm doing well in school (got a larger dose of adderall for my add). I was even seeing this one extremely cute girl a few times this past two weeks, first time hanging out with a girl in over a year. It was the longest period of time I can remember were I didn't feel depressed. Then this past week I got sick. No big deal first couple days. I couldn't workout, felt groggy. Then depression came back. It didn't hit me hard, just gently crept up out of nowhere. I tried to push it out of my mind, distract myself with schoolwork, but was still sick. Yesterday got really bad. I was in a weird familiar state, feeling like an observer of myself doing everything. Making food, walking to class, talking to people. Today was really bad. Skipped all of my classes. Got out of bed at 2, walked into the bathroom and looked into the mirror and started to cry. Then sob. I was legitimately surprised how it caught me off guard like that. Dumbfounded. When I was feeling good I almost forgot what it felt like. I can be happy, then depression will just whisper things in my ear out of nowhere and drag me back in the darkness.