This is a difficult and sad situation, and I can understand why it would feel so bad. Can you recall what it was that you were hoping for when you went looking for information about your mom and planting the tape recorder? It might be important to understand your motivation for wanting to know and how that might be influencing all of this.
I would talk to your dad about seeing a therapist who would talk to you individually and with your father. You’ll need a third party to sort through what has happened, what is happening now — and making plans for the future.
Make this a priority discussion with your dad. He can’t continue to use you as his emotional support during this and you not having support with a therapist. Please ask for this ASAP.