My transaction is still in mempool for 3 months now ...
I assume you're talking about this transaction:
42888c7b1540f8d5b9e074d15c2d86d7b03878adf6f173c7316f09910c2dabd9.
Why is it not getting out of the mempool?
There is no "one mempool". Anyone can keep that transaction as long as it's valid. Easy fix: move one of the inputs.
For example, (check and if you like it) sign this transaction:
01000000010481a776ff31478b8b967e15a9e68f78f4cd06b156a5d12910f72d8885147e95020000001976a914367ca154148ceabbcfc3b6c6167f512bb4f9c23388acfdffffff01b88800000000000017a914e09aa133479cff513578a460c606ba504fd761a68700000000
This takes your smallest input, adds a higher fee, and sends it to your destination address. I used
coinb.in to create this unsigned transaction.
Once this is confirmed, your months-old transaction is no longer valid.