Does your tongue tense as you sing? Does the back of it pull up as you go for a higher note? Does it push down into your throat? (That will cause your voice to get tight and lose tone.)
Here is a quick tip to relax a tense tongue. Do this as an exercise, not when actually performing.
As you sing a song, rest the tip of your tongue behind your bottom teeth. It will need to move a bit to accomplish some of the consonants, but otherwise, especially as you sustain long notes, leave it relaxed low in your mouth with the tip resting against the back of your bottom teeth. It may take a bit of practice to break the habit of tensing and overworking your tongue.
Remember; do let it move for the consonants so that your words will be understandable. But you will discover that releasing tension in the tongue has a lot to do with making singing easier.