It's awful disappointing to slip down after an extended period of feeling relatively well. (I've been through it.) I think the key that you've identified is the loneliness. If you can possibly draw on the human contacts you have for more support, try to do that. Also, try to expand that support circle.

Being at a lecture is like being alone because you don't really have to interact. In a way, I think it can feel more lonely because you are surrounded by people and yet you may be uninvolved with them.

If counseling is available at your school, check it out. It is reasonable to expect that this slip back will not be permanent. That you went a good while without feeling bad shows that you are resilient and do have the capacity to right yourself, emotionally. It can get easier with some moral support from someone you can trust. I hope you have someone IRL to turn to. Meanwhile, you have us here. Let us know how you are doing.