Firstly, learn all the major scales perfectly.
There are 12, but you only need to learn half of them. And they can be grouped - i'll show you what i mean:
1) C major - all natural
2) G, D and A major - memorise the sharps
3) E and B major - memorise the naturals (the others are # e.g. E major = E A and B natural; everything else is a sharp).
The other scales can be learned with the above scales as a mental guide.
I'll explain below.
Db major isnt as scary as it sounds. Think of it as with D major scale. D major scale = 2 #'s: F# and C#. For Db major, the two sharps of D become the two naturals of Db; all others are b, as it is a flat key. So the two naturals in Db are F and C (other notes are b). So, for the 'D' scales (Db and D), just remember F and C.
E major = two naturals (E and B). Eb major - the naturals of E major become the b's of Eb major (and the others are natural).
This may sound a little complex, since i'm writing it quickly here, but it is one of the easiest ways i have found of teaching students on how to learn the major scales from memory.
This should come first before all other theory.
PS i'm assuming you know the major scale interval formula? This is where the notes from the major scales come from. There is a fixed pattern that yields the notes for each scale (the same pattern for each scale).
ALL other scales/modes can be learned relative to the major scales (e.g. lydian = major scale with a #4 etc etc) - you don't have to learn a full new interval formula for each new scale/mode.
Unfortunately many 'theory' books still try to teach the interval formula for each scale (i'm sure the ABRSM uses the 'whole-half' interval formula approach when teaching the minor scale, instead of saying 'flatten the 3rd, 5th, and 6th degrees of the major scale.) All the information is correct in these books, but what is wrong is the way in which they encourage you to approach the theory, mentally. Hence you have probably had problems with the books you are trying to learn from.
You should quote specific examples of what you are tyrying to learn (and from specific books, if possible) - this would be easier than saying you are trying to learn 'theory'.
One final thing - you should call this kind of thing 'harmony', rather than 'theory', as we are dealing with the harmonic nature of music here (e.g. how notes relate to each other). 'Theory' could be learning the meaning of 'sprechgesang' 'ostinato', etc etc (think boring lessons at school - this is theory
)
i'm still learning!
