Yet again we find another parallel in our lives. My wife has the voice of an angel and in an effort to support her in that I become a part of our church music team. I have practically no musical ability or timing. Given my perfectionist nature this lead me to 'sound guy' rather than up front singer. Then because I am a natural leader I ended up co-music co-ordinating with my wife. If I can't be 'perfect' at something I won't do it. The thought of someone cringing at me not hitting the right note or coming in at the wrong time is mortifying for me. Like you I know where it comes from. I do not sing in public....but I do love to sing when no one can hear me.
I am happy to be in front doing drama....but singing...No way!
I do commend you on your efforts to support your wife and be involved in her 'stuff'.
It will get easier to follow as you attend more practices. I am sure others are not as harsh on you as yourself (or your stepdad). So maybe stick at it for her sake. The other option is to cop out like me and find some other background way to support you. You do have to pick your battles.
More than meets the eye!