Look in the mirror. If you see a saint, there is something deeply wrong with yourself and your self-perception. There is no such thing as "being a saint". Being human means having a dark side, that which Carl Jung called the "Shadow", that which grows stronger if repressed. Being human means having pervertions, feeling hatred or envy. There is nothing wrong with those things - on the contrary, they are perfectly normal, but they have to be incorporated into your life, or else they will destroy you.
In this sense, "being nice" is not being nice. It is playing a character, putting on a facade, performing an act - usually with the subconscious expectation and pretext of getting something back for putting everyone else on a pedestal. Also common, the belief that you have to be nice and upset noone. In reality, you're just a manifestation of a man who was brainwashed into being meek and, in the process, you lost connection with who you are. You grew up to the Disney utopia.
Maybe you put everyone else's opinions and priorities above your own, thinking that there is some kind of universal karma that will pay back to you in the form of a pussy who will love you forever and unconditionally. Because your perspective of the world is so delusional, you think that everyone else is just like you - special snowflakes - and that that girl who was too busy to go on a date with you must be telling the truth.
Your empathy is rewarded with contempt.
Eventually, there comes a time when you realize that something is wrong with you and, when that time comes, every resentment you ever denied feeling will punch you in the face. Your awakening will make you collapse. You will be not so nice then, will you?
This is the most important step into true masculinity. As Jordan Peterson puts it, "turn into a monster, but a civilized one".
"You gotta let them know what kind of guy you are, so they know what kind of girl to be. - Ken Cosgrove, Mad Men
A nice guy treats every women as a virgin princess. That's his frame. That's how you want them to be, so that's how they are to you. It's a self fulfilling prophecy.
Get in touch with yourself once again. Realize the darkness that lingers inside you and embrace it. Redirect it. That's your edge - use it wisely.