My Actual Big Question

My original big question was something I grown uninterested in because I feel like I already have an answer to it and just chose it because I had nothing else. I hadn't found a question that applies to me until recently when I was looking through an old journal of mine. So my inquiry is, "How do we get over people telling us we can't do something in life because we're "not good enough or it's impossible" and rejection or fear of rejection?" Will things truly work out even of you work for it?Like adults told us when we were younger that we can do or study anything we want to in life, impossible is nothing, and follow your dreams but once we get older, they kind of force us to do what they want us to do" To me it is so fustrating when my mom asks me what do I want to do or study but then answers with  I can't, it's a waste of time, or is impossible. "I should use my time to study to be a nurse or dentist and not waste my time." Why are (most) adults so supportive and encouraging when we were younger but suddenly change their opinion? Almost as if they were lying to us and are trying to run our life. This may not be a philosophical question or anything of that sort but it is just a question that's been getting under my skin lately.