If you are having a hard time, I think you are in no position to be picky about methods. Everyone learns differently. Try this and let us know how it turns out
Some people think in numbers, some think in solfege, some maybe think more in instinct or visually, over time they're learned, maybe subconsciously, where to put their fingers relative to the current note to make the sound of the more common intervals: fourth, fifth, etc.

I always found it easier to think in numbers then solfege. "5" just seemed more intuitive then "so". Especially when you start throwing in the accidentals. But hey, whatever works for you. The overall objective is the same: relative pitch.
There really aren't any easy ways for ear-training, but it might help.

Actually, in terms of fretboard navigation, that lesson began in a direction similar
to the Guitar Grid method which is actually pretty good. However, it didn't really
explain enough to be workable.

I wrote a similar method on how that works here: