I would try to teach myself to wait until the other person I'm handing the whatever to makes the first comment. Or, I would simply try to change my "it's not very good" to a question like, "How is it?" or better, "How do you think it is?" and try to respond to the person instead of to whatever it is that you have made. Think "person" not "thing". If you can focus on another person then you won't be able to focus on your production and putting yourself down (because you're probably not going to call the other person a liar when they say how good whatever it is is :-) Pretend you're entering whatever it is in a contest and you aren't one of the judges so what you say doesn't "count."