Sounds like a worn barrrel, as such its probably not engaging the central locking and is therefore not unlocking the car.
Your going to need to replace the barrel at a guess, you can buy them off ebay for a lot cheaper than Ford, and you could then use a Haynes manual to do the job. In comparison to other jobs, its tricky but not impossible for a 'rookie' to do, so rate your own ability and go from there.
Its also worth waching youtube for changing Ford Door Barrells. There are a few tricks and tips they can teach you, there is also somewhere a video for taking a new barrel and setting it up for your key, so you wont need to change key (certainly to my knowledge).
Does the key start the car without issue and resistance?