Pfew… No more mister Nice Guy, but then from a more psychological standpoint. Going to be a LONG night here, but if you need some advice I am always willing to help people out
i have walked that road you are talking about, and trust me, you need a big push to help you with it.
I am going to assume that you are the holy jesus himself, the most religious guy in the world, Mormon or anyone who has never been outside his own community. Don’t have to correct me, but that’s what I am working from. So say that you are that person that cannot lie, that has so much trust in his neighbors that he will keep the door unlocked at night and does think he will never get shot, robbed or anything will ever happen to him. Goes to church every sunday, puts a lot of money in the donation jar and even helps a poor granny to walk over a very busy street. That is what I call “mr Nice Guy”.
Now, you want to change that person into a person who can say no, is a bit bossy, uses his elbows to cheat his way to the top, curses at the government for doing nothing, never goes to church unless he is taking pictures, wants to gamble in Las Vegas, if necessary will drive someone over, and starts shouting when he does not receive a Mc. Donalds menu in the next minute.
To get from A to B, you are going to do a lot of things. First of all, start to say no or make an excuse why you cannot do it. I know, it sounds really awful, but try to cheat your way out of things. People will keep asking you stuff and you have to set priorities. Start taking back control over yourself. If they dictate your life, nothing will ever happen.
Read the news, and especially the politics. Study them, learn from the masters. What do they do to cheat their way into the top? What makes them earn twice your yearly salary in a month time? How can they get to the top? Learn to curse them, form your own opinion about stuff instead of blindly following them. In the Netherlands, they had over 25 different political parties during the election! Politics is not Red or Blue (or a team deathmatch), politics is making laws that will affect you as a citizen while making them rich. Form your own opinion and see what your “parties” think of it. Trust me, you will stop voting for that party that your whole family was voting for 
Play games like Postal. See how long you can survive without grabbing a gun and shooting at people. Yes, it is possible to do so, but it is agonizing slow and shows you how far you come when you are nice to people. Yes, you can be nice to people, but look at the bigger picture. Are they actually honest to you? Why do they want your attention? Are they doing that for their own personal gain, or are they doing it because they genuinely like you?
Look closely at the stuff you are currently doing for other people. Is it really worth your time? Does it give you any gain, not only financially but also mentally? If its sucking all your free time but its not rewarding or anything like that, why are you still doing it? Most likely they push that stuff to you because you don’t complain or say no.
Look in the mirror, and tell yourself how fabulous you look, how handsome you are and that you are the man of the house. Think about who you are, what you can do and why you are born. That person staring in the mirror is the one that must get all the attention, not the others. Stop bothering other people if you feel they don’t need you. I lost a couple of friends who never called me anyway, and I did not feel any different about it. It did not make me feel sad. They did not care about me anyway.
And lastly, get the anger out, one way or another. When I go karting, I want to be the guy at the top. And all those guys bumping me or trying to overtake me, I will just bump them back to where they belong: Behind my ass. During laser gaming, I am the guy that is telling those kids to run, to get them hyped up and to push them to their limits. Trust me, after 2 sessions with those kids you are gasping for air while they are demanding another game.
When you are at a coffee store or anything where they serve your food slowly, start asking politely but a bit aggressively how long it will take. Don’t wait, ask. In the Netherlands, staff is generally less polite than in America where they live on tips. We give our waiters a fixed income, tips are generally something you only give if you really love the service. If you don’t push them in that feeling that YOU are a client that is paying THEIR income, then they will see you as another ghost at the table.
I was also being mr. nice guy when I was very young. Even got diagnosed with mild autism. Its thanks to my family and my close friends that I actually found out how to deal with this. I still like to help out people, but I also found out that if I let the wrong people dictate my life, I would actually get depressed. First learned that fact with my ex girlfriend, she still is trying to contact me but I moved on…
If you like some more advice, or like to talk to me in person and learn a bit about how to become less of a “mister nice guy”, I am always happy if people visit me sometimes. Makes me less lonely and I get to see a face with the nickname 