I’ve heard a lot of times that when mewing, you’re supposed to be pushing up, forward, and to the sides, but when I meet, I can only do it in one of those directions and never all at once. Does anyone else have a similar issue or know what to do about it?
Yep you're doing something wrong. Most probably the lack of tongue strength and dexterity, combined with very low tongue space. Or poor head posture. Or a combination of these.
When you mew, your head should be in perfectly on top of your body. When this is done, your hyoid and hence tongue, is perfectly below your palate. Now when you put the tongue in the right position, it is automatically being pressed right up against the palate, due to the lack of space. When this happens, thr tongue spreads out like a ballon that is being pressed against a wall, and this spreading out occurs in alk directions, hence forcing the palate in all directions equally.