I have a student who freezes up reading rhythms. He has dyslexia, but is very musically talented and accomplished (working on his master’s in voice).
One crucial aspect of learning rhythms is the FEELING. Not just the thinking, the studying, the intellectual knowledge. Rhythm needs to be felt and understood in the BODY.
Physicalizing rhythm – putting hash marks for beats in the music, learning to tap dance, getting a pair of drumsticks and a percussion pad – these are all helpful possibilities, and go beyond studying notated rhythm per se.
The parts of the brain used for “counting” (with words, syllables, or numbers) collide with the parts of the brain used for lyrics (words again). This is part of what makes singers seem more rhythmically challenged. In a person with dyslexia, things get even more confused. If rhythm is a “seen and felt” phenomenon, then it doesn’t have to be “counted” as much and it won’t collide so much with real singing.
For things like off-beat quaver (eighth) rests, get the body moving, even if only the toe is tapping, and learn to feel which part of the toe tap the rest is on. Et cetera. Find yourself a Dalcroze or Orff teacher. This is their bag.
Intellectual study alone will not reduce the angst and get the rhythm integrated into the music.