You're not useless!
I did some scales in thirds, can't play grade 5 pieces

but scales I am reasonably good at, so my teacher gave me some so I was at least progressing in *something!!*
One of the things that helped me, was to start the RH on the keynote, and only start playing with the LH when the right hand hit the 3rd, eg in C major, RH plays C, D, then LH joins in with C when RH hits E. Other way round for 6ths. It feels (or at least did for me!) much less unnatural in the RH then.
Also make sure you're 100% confident with the scales hands separately and one octave apart so you have as little as possible to think about beyond the 3rds bit of the equation.
Took me *ages* to suss them but it eventually clicked. You're light-years better than me on the piano, so I am confident you can do them!!