I direct A Cappella Choirs and maintaining pitch can be a problem.
I don't have a problem myself and some call me the "Pitch Doctor"!
I suppose that's one of the reasons I'm the choir director.

Really listening to what you're singing slowly [each note] can help. You may need a keyboard to do this. It's a question of remembering where you started from, and locking into that.
Learning to sing a semitone scale is a very good exercise. Also a whole tone scale will really make you think about where the notes are.
You could try a few singing lessons. You have to find the right teacher though.

I like the comment re opera singers and vibrato! I don't like vibrato, and ask choir singers not to use it. Very few even try to sing with it. This is in community choirs where most have not had any voice training.