I am practicing triplets on guitar. I can play them at a decent speed accurately and I emphasis (alternating up/down strokes) the first of each set on the beat and can feel them internally up to a certain speed. If I up the speed then they start to feel more like even tuplets. or 16ths etc. Is it essential to keep feeling them as distinct sets of 3, because essentially when practicing like this, they might as well be 12ths of a 4/4 bar. I can't imagine still being able to feel distinct triplets when the speed gets high.
Downstrokes with a pick are generally more emphasised than upstrokes. Playing triplets, and alternating down and up strokes, the second and fourth beats start with upstrums, which need to be as emphasised as the downstrums on beats one and three. DUD UDU DUD UDU in each bar. I suspect that's what's throwing it out of kilter.
I think you may be mixing up compound meter and tuplets.
...12ths of a 4/4 bar
I think you mean 12/8 meter...
...which is sometimes written using tuplets like this...
But I think the normal meaning of a tuplet is that it deviates from the basic metrical subdivision, resulting in a mixture of subdivision of the beat, like these...
If you are practicing constant subdivision of 3, then it seems you are really playing a compound meter feel.
To really work on the feel of tuplets, you should alternate between the basic meter like 4/4 and then throw in a few tuplet beats divided by 3.
FWIW, I used a metronome to practice tuplets, because I didn't trust that my tempo would not drift. And it was harder to play them slowly.
A good help to stay in time with triplets is our language. Instead of counting or using syllables is to say words that are spelled in a way of triplets like Mexico or other names of countries. But take care: if yous say Canada you can fall in the other rhythm of two 16h and a 8th note again.
Another practice is to use play the rhythm pattern strumming with different fingers of the right hand (Guitar):
e.g.: 1 (all fingers down) 2 (thumb down) 3 (all fingers up.
You should also practice the triplets without Guitar. How about the Ravel's Bolero?
(tapping 8th notes with the thumb and the triplets with fingers 4,3,2