Why do I still have depressive episodes after meditating for so long?

I've been meditating faithfully for 4 months. I was promised more peace and resilience. But this week, a depressive episode hit me just as hard as ever, and all the negative thoughts came rushing back. Name: Akash, 31/1/1999, 10 PM. I feel so discouraged and almost cheated by the promises of mindfulness. Why didn't meditation prevent this? Does it only work for mild anxiety, not for real depression? How long does it truly take to see a difference?