Do you know what is wrong with my gearbox?

I have a 51 Mondeo LX Auto. About six months ago I lost 1st and second gears in my D mode. I then continued to use the 1st and second drive manually. I now have lost the second gear, the first gear still runs but sound like it is slipping. If I can get up to 20Mph I can slip it into drive and pick up thrird gear (just) and it will drive V well. A friend of mine has suggested it may be something to do with cooling as my engine never seems to get warm (at least the heater doesn't) but then nor does the thermostate guage. Any suggestions?