You can't switch to simple recovery with active database mirroring. You'll need to completely drop the database mirroring setup, then you can change recovery models. Recreating the mirror will require copying over and restoring a full or diff backup. That's a fair bit of work and you'll be completely without the mirror while that's done.
We do daily backups at 11pm, a diff backup at 6am and hourly transaction logs from 7 am - 6 pm - plus the mirroring.
That's probably part of the cause of the large log, the lack of log backups for 13 hours a day. I suggest log backups on the hour, every hour.
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: Discussions on DB performance with occasional diversions into recoverability
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