1997 GMC Jimmy gear slipping

Alright, I'll admit to being a dummy, and admit to taking poor care of my vehicle, I have my reasons such as they are.

but, my truck has been running just fine for months, then we get a cold snap, it's still only 12 F outside!

I immediately had trouble with the shifting. Reverse, first second, all work fine. But it refused to shift to third. And it still won't go into third.

after checking the ATF and noting that it was, indeed critically low, I systemattically added more ATF rechecked, added some more, and now, it looks like I have almost enough, to where less than half a litre will bring me to where I need to be, I think. However, the problem did not alleviate, as I added fluid.

how serious is my problem, what risks, are there, and what options do I have to remedy this?

hope you can help, and hope I didn't drive my truck into the ground, after all, I did top up the ATF in Sept. Perhaps, it NEED more frequent attention? I wonder if it might lose fluid faster than I was lead to believe.