I think what I have learned since starting this thread is that while I sing a song, even within an octave and a half, the highest and lowest notes are strained. I think it depends on the intervals more than the particular note - jumping from a low note to a high one is a strain, and it feels higher than it is. I found today that if I sing some scales and practice the intervals in the song and others, before I sing, it is much easier. I guess what I need most of all is just lots of regular practice, which I will make sure I do. Making sure to breathe fully before a high part also helped me considerably.