Oh, those wretched plastic mesh inner pots. I had a house plant die in one of those. It was only when I emptied out the pot that I discovered its roots had been choked to death.

And, of course if you just tip the whole thing into the compost, then you end up with plastic in the compost.

You may well have been overwatering. Though hard to say without actually seeing it.

Parsley won't do as well long term as mint. Mint is a perennial, parsley is an annual. The usual problem with mint is holding it back from world domination!

If you have a compost heap (and if not, why not?) then put compost round your new plants. They won't have much food in the 'soil' in the pot.