"I’m very aware that I’m viewed as not successful enough to have an opinion on anything. I just want to know why and what I need to change to at least be spoken to as an equal, or even better as someone to value the opinion of."
Your opinion matters, it's just that some of the ignorant people around you don't acknowledge it, and that's shitty, ignorant and arrogant of them. iI also smacks of sense of superiority from them, which is often underlaid by an acute sense of inferiority - people who feel inferior often lash out and bully or put down the people around them because they think it will make them feel better.
If the debate is about, say, how best to build a bridge, and their trade is bridge building engineer, then maybe they could adopt this stance, but for pretty much all other conversations
a) you're entitled to an opinion (esp if you've read up on something, have personal experience, or 1st hand knowledge , or just an opinion based in being human being who's lived x many years
b) it's bloody rude to shut someone down like that and
c) adults debate and discuss things from different viewpoints to expand their knowledge, experience and data on a variety of subjects all the time.
d) if they listen they may just even learn something. (My guess is that's what they are probably scared of, I mean how humiliating that you a mere whatever you are , is telling them something they've never even thought of ! The temerity of you being cleverer/ better informed / more imaginative , don't you know your place?
Stand with your head held high, tell them to stop being so rude , ignorant and disrespectful. And if they continue to treat you this way tell them to fuck off 
Seriously. You deserve better.