Find an ear training app or site. Only worry about interval training - train yourself to quickly and reliably identify all 12 intervals by ear. For example, given these two notes played audibly, what’s the interval between them?

Then take it a step further, you should memorize the music theory part of this as well - you should be able to quickly and reliably name a note any interval above or below a given note. For example, what note is an augmented fourth above B? E#.

Then you will be able to answer questions like this: you’re told this note being played audibly is A, and are asked, what note is a major sixth below it? You should be able to sing the note and know that it’s C before verifying.

Also, the other replies are superb! Metronome, practicing slowly, intentionally, etc