I think @Tim has the right idea - use your instrument to help you - the music itself will teach you. It always does, if you let it.
If you are having trouble with certain intervals - practicing scales, chords and arpeggios on your instrument with those intervals - counting off the intervals and naming them, and then comparing their sound with other scales that differ only in the interval that's giving you trouble - should help you a good deal. Singing out the interval names as you go can also be helpful to fix the sound in your mind: "R-O-O-T | M-i-n-o-r 2nd | M-i-n-o-r 3rd"...
In the case of m2nds, try working through Phrygian mode for example and comparing its sound and the sound of its arpeggios, to natural minor (Aeolian) in the same key - the only difference between them is the minor 2nd in Phrygian.
Try working through Dorian mode and comparing its sound and the sound of its arpeggios, to natural minor in the same key - the only difference between them is the b6 in Aeolian.
Try playing a major scale and comparing its sound to a Mixolydian scale in the same key - the only difference is a dominant 7th vs a M7th - focus on how and why their sound differs because of the leading tone vs the dominant.
Try playing a major scale and comparing its sound to a Lydian scale in the same key - the only difference is the augmented 4th vs the perfect 4th.
Taking it to the next level, absorb the sound of different pieces of music that are exploiting the m2nd in particular. @alephzero touches on that in another answer.
By focusing how the minor 2nd functions and sounds in different musical scenarios, in contrast to a major 2nd, you will become familiar with the particular sound and quality of the minor 2nd. Same applies to all intervals that are difficult for you to distinguish.
BTW: Three days is not a long time! Many times you work on something for a while in music - more than three days - and it doesn't seem like you're making much headway. Then one day, everything comes together and you discover it has become easy.
Also, when you feel like you're at a dead-end, take a break and do something else, then come back to it. Same thing might happen - after the break, everything has jelled in your mind. The brain is funny like that - it doesn't usually like to be forced.