I’m guessing this is the video @kwt7667 is referring to?
My best advice - don’t give up!
My second best advice - come up with a melodic reference for each interval. So if you’re working ascending major thirds and perfect fifths, you need to associate a song with each one.
Like, for major third, you could use the first two notes of When The Saints Go Marching In (“Oh when”).
And perfect fifth, you could use “Twinkle twinkle” from the children’s song of that title.
So then when an interval plays, you just try singing the words from both songs and see which fits.
This is a great page for finding songs to match intervals -
Third best advice - try tweaking the exercise! You could try major thirds and minor thirds, or fifths and octaves, just mix it up until you find a successful starting point to build from. And don’t quit!