Hi Moon, thanks for posting.
I feel your pain - I am not good at dealing with negative feedback, either. But something you need to remember is this: music attracts people with massive egos because it's one of the few paths to fame and wealth in this world that's open to the average guy in the street.
A lot of people reviewing a song will be stuck in a competitive mindset rather than in one that's open to constructive criticism. If the people who left negative comments were unable to back them up with anything (eg. I hate it) then simply ignore them. If someone says "I hate it because the chords were the same all the way through and there was no definition between verse and chorus" then you have something to work with.
It's possible that your piano playing is not as good as people were expecting. It's possible that you have old or cheap equipment that just doesn't produce a high quality sound that people sometimes expect.
All of us have room for improvement, especially with regard to singing, but I would say don't let comments about it be a reason to take too big a knock. Anyone who's got the ability to be honest with themselves will know what their strengths and weaknesses are. If you've asked for criticism on your songwriting, and been given negative comments about your singing and piano playing, then that's not what you asked for.
It's one of the reasons I created this forum. I encourage people to be honest, but never rude. If you have an opinion about something you should be able to back it up with something that person can work with. The ego has no place around a community of creative folks like us.
Try to roll with the punches. Decide what criticism is just people being idiot, and what's a genuine reaction to something you need to improve.
If you want feedback on the feedback you've been given, I'm happy for you to create a post with the song and the comments you've received, and see what our members make of it.
Take heart my friend - you have my support