Best advice I can give you, one singer to another: Practice.
I am only a Soprano when I'm using my falsetto voice, but otherwise, I have the same problem. Try to sing the scale in one key and go up key by key and when you feel like you can't do it anymore then try going down as low as you can and then again up. BEWARE: Your voice will get very sore when you do this so definitely have some tea or water on hand when you do this.
To you second question: YES! Don't worry about the hecklers as long as you love what you do and except that there is ALWAYS room for improvement, you will go far.
Best of wishes to you. Hope you go far and that this helped! :)
