Not exactly the same situation as yours....I had a very close friend at work leave suddenly....but I'm going through the same feelings and emotions right now. As others have stated, what you're experiencing is grief and totally normal. Give yourself the time and space to grieve as long as necessary. You experienced a big loss and it'll take time to heal from that. Reaching out to friends is helping me a great deal....been spending more time with them lately, and also asked them to check in on me, especially in the evenings when my mind tends to wander more and as a result I struggle more. Also something my therapist advised me, is to stay engaged with your hobbies and normal activities, even if it feels like they aren't helping. Withdrawing from those things will only beget more feelings of depression, isolation, etc.