Kathy, I went through a difficult period time last year in trying to fix why my 1/4" seam was not a 1/4" seam. However I was determined ( and tearfully frustrated) and it was a combo of things. First, I had to find my 1/4" spot on the sewing machine and used 3-4 layers of blue painter's tape.
Secondly, I slowed down sewing. I was working on several log cabin squares and I had to rip out seams way back. I too have the June Taylor slot ruler but discovered it wasn't accurate. I measured every time I created a seam to make sure I wasn't 'widening or narrowing my seam allowance. Third, I had someone check my work and offer suggestions.
Fourth , I measured every time I added a log. I persevered and watched you - tube videos and bought a new 1/4 in foot with no guide. That by itself was a big help but continued to use the painter's tape. I think you need someone to assess your situation while you are sewing. I still have to careful and patient and ever so watchful ! Sometimes you just can't tell what is happening.
I hope this helps as I know exactly how you feel with this - it's a skill that takes time and care. And I am still working on it. Keep on trying and don't give up