Didn't read the entirety of the thread but I'll chime in what I learned.
I used to be the "Nice Guy" with the Nice guy Syndrome and all that. Subconsciously demanding the world "why women don't like a nice guy like me".
These last few years I start to realize that I like being nice. Like true nice. Or rather, I like being a good person.
My suggestion and, dare I say, word of wisdom, is this: instead of being the "nice guy", either be yourself completely without feeling the NEED to be nice or polite, or if you're genuinely nice, become the "Knight/Samurai" or whatever you like to call instead. What I mean is the archtypical knight or samurai: noble, honorable, polite, a gentleman, but aim to greatness with moderate ambition and can stand up to rivals and enemies and "kick ass" if the situation really calls for it. I believe it's the next evolution of the true nice guy.
What is left is really for you to think if you're being nice for your own subconscious selfish gains, or you want to go down the noble path and be the Knight?
And never be afraid of rejection. If a warrior gets knocked over, he gets back up to fight, over and over. If you keep lying down, you'll just be finished off.
EDIT: I forgot to say I hope it helps and good luck to you in the future! I figured these out on my own in three years, and I hope by sharing it, it'll take you much less time to reach that.
Also, seeing that you're a Troper IIRC, look up Yamato Nadeshiko. The "clear touch of iron" is what I want you to think about.