How should I learn E in Morse code?

Listen to dit as one rhythm at 20 WPM rather than counting individual marks.

International Morse alphabet

E in Morse code

The shortest letter: one dit

.dit

30 second sound drill

Listen to the complete rhythm. Do not count its parts.

Repetitions

Ready

Hear E as one sound

At 20 WPM, the elements join into a recognizable audio shape. Farnsworth spacing leaves time after the complete character without changing that shape.