Hmmm....
Work with me on this because I'm not too terribly familiar with Mercury's, but if you say that you were continuously having to refill the radiator with Antifreeze and now you're having engine problems, the one thing that is coming to my mind is the possibly that you may have warped something due to the inconsistency of coolant running through your system. The only reason I say that is because, while you did refill the radiator about every three to four weeks, that's still assuming it had JUST emptied at the time you refilled it. If not, then you were running your engine hotter than it should have been running.
If you think you have a radiator leak, which it sounds like you do, the first thing I would do is look for some radiator stop-leak. It's something like 8-9 bucks and should help until you get the car looked at. Still, I would definitely take the car in to be serviced so as to not damage the engine anymore than it might already be.