06 Hatch does not like backing up

[microtandme]

my 2006 2.3l mazda 3 hatchback automatic does not want to go into reverse. Normally when it has been sitting for an hour or so and its cold when i start the engine and go into reverse i hear and feel as if it is shifting into reverse but it does not kick in i was told by my mechanic (buddy if mine) that it just needs a transmission fluid change the fluid was black thin and smelt burnt so i changed it seeing some results but problems are still there. When i start the car it will go into drive perfectly but not reverse. i just (10 minutes ago) found out if i hold the break for a bit while i put it into reverse i will feel a little clunk then shortly followed by a big clunk thatll shake the whole car then it will back up fine.

might need a new transmission but hopefully not...

any ideas? anything would help thanks!