Move from that vowel to weak vowels. Vocalize back and forth, strong vowel, then weak vowel. Practice a song this way. Practice in varied acoustical environments. Practice in performing halls whenever possible.
Late nights are good times to find performing halls empty.
Get a friend to repetiteur for you. Get coaching from a professional voice coach. They listen with different ears from your voice teacher and can add a great deal to your performing ability and to your interpretation of music. Set regular times for practicing and keep it a top priority. Do not let other activities replace these practice times.