how come i literally for the life of my ability mix these two up in the major key context. do, re, mi, fa , ti no problem, but so and la perplex my ear training. will i ever be able to hear the difference?
“So” I’m guessing refers to a perfect 5th, and “la” is a major 6th. For me, if it sounded like a power chord I went with “so” lol. “La” always sounded a bit more “major” if that makes any sense. Hope this helped a little bit
Get a teacher and you will nail it for sure.
Just sing the beginning 4 notes of twinkle twinkle, but replace the words with “do do sol sol”
If you do that enough it will cement itself in your mind’s ear. At that point you’ll know “sol” like the back of your hand. “La” can at that point just “the other one”
just go between DO SOL and DO LA and use your hand signals if you know them which im assuming you do. or go between all of them like DO RE, DO MI, DO FA, DO SOL, DO LA, DO TI, DO DO 8VA.
try that for a little bit
Try these exercises:
- Pick a random note 
- Sing the So or La below it
- Pick a random note
- Sing the So or La above it
When doing these two it’s important that you’re not thinking “to get down to the So I have to sing a perfect fourth down”. Instead the exercise is about maintaining the Do in your mind and then producing the harmonic pitch that spells out the La or the So interval. To phrase this in another way, you’re teaching your mind how the La or So sounds in relation to the Do instead of what the interval of a perfect fourth, or minor third, descending sounds like.
- Pick a random note
- Sing the So or the La and resolve it upward in the major scale to the Do
The goal here is to associate a feeling of movement and instability with these intervals along with a different path toward resolution. I’d be a little bit careful with doing too much of exercise 2. I did a lot of this and found that I would sometimes automatically pre-hear the major scale resolution of La->Do even though I was working with e.g the Dorian scale. Thankfully it didn’t take long to unlearn this. Perhaps you can mix it up and sometimes do a La->Do in major, Dorian, Mixolydian etc to work around this problem.
If you’re into tools them EarMaster (custom exercise, interval singing) can help you with exercise 1 and Functional Ear Trainer v2 (miles.be) will help with exercise 2.
Best of luck!
 When I say pick a random note I mean to ‘tonicize’ this note. Depending on your ear that might mean just play that note to make it the ‘root’, or it can mean play the note, then the triad, then the scale, or play the note and then e.g. a I IV V cadence. Just do whatever cements this note into your mind as ‘home’ for the exercise at hand and try to work down to being able to quickly tonicize a single note after hearing it once.
Julie Andrews is the model for how vowels should be sung.