Here are a few thoughts. Do other people think you sound nasally? Have you tried recording yourself and listening to the recording? What you hear as the sound resonates in your head is not what other people hear.
Another thing is that you are probably straining to reach those notes at the top of your range, which causes you to close your throat, exactly the opposite of what you should be doing. Open your mouth wide. Don't try to sing the long e vowel or the short i vowel. Stick with "ah" and "eh" as much as possible. Even leave off some consonants if someone else is singing a lower part.
When you say that your range only goes to a high C#, do you mean that you can really sing up there or can you barely squeak it out even if you are already singing high in your range? Maybe C and C# are not really in your range.