How can I tell the difference between what my ego thinks I want and what I truly need?

Want vs need. So much that we think we desire is superficial and a passing phase. What we need is a definite , categorical need. Much is spoken about knowing what we want...but sometimes when you get what you think you want it isn't actually what you need at all. I can only suggest taking time and consideration when manoeuvring in the world. I believe we know somewhere deep inside what it is we need, but it comes with maturity, and when you are not being swayed by other people's desires/acquisitions...
Ego is a big little word, so much is dictated by this maelstrom of a human facet...it wants it all! Sometimes you have to squash the fucker into a box and be realistic, but it can be such a driver as well, so let it out at appropriate occasions and let it dance, but don't let it dictate YOU!
I think life is just a series of trials and errors, a constantly learning, dare I say journey. It is NEVER too late to reform and reassess. Go with your heart, and if it isn't right, scrap it, forgive yourself and start again. You are always learning about yourself, just try not to be too fixed in your opinions.