OK, well it sounds like you might have acted in ways that are not proper in the past and now you have guilt and shame over it. Like you said about yourself > selfish and greedy. Well maybe you have been that way? Im not sure I can only speculate based on what your posted

>>> But you can change that.

Thats the good thing. If that is true about you > at least you have noticed this and now what you need to do is Change that about yourself if that is what is making you unhappy.
Change is Possible. But you DONT change that by hurting yourself.
You change that by makng an adjustment to your way of thinking and your way of acting in relation to other people. Life does give people the Opportunity to change.

All of us have been rude or selfish or hurtful towards people at some point.
But you can change. You just have to be committed to change.
When I was a child I was mean and nasty to people and then one day I woke up to that fact
and I was ashamed of that then I did some inner searching and realized I needed to change that and I did.

Now I get along with everybody and make friends easily and interact with people positively
I changed. We all can change. Going through changes is part of the human experience!!

so stop thinking that your a nobody. Your NOT A NOBODY!
Your are somebody just like everybody else and equally important too.
Your just coming to a place of understanding that you need to change some things about
yourself and You CAN do it. Work on it.

Start a journal and write down things about your chracter and your life that you want to
change and work on it. Keep a record of your progress and also realize that sometimes big changes dont always happen overnight. But you can start making little steps.
Plan to make changes that you need. Stir up the Desire to change those things
work on it. Next thing you know a year will go by and you will be a totally different person
a person who gets along with others and has a good social life and has opportunities coming your way. You can do it.
For me when I was in a big stage in my life of changing my life
I came up with this Acronym I posted on the wall and even wrote it on a T-shirt I would
wear smetimes. It was
FIASOI
It stands for >> Forever In A State Of Improvement

So I had it on my wall… to remind myself each day to Improve upon myself each day
and when I wore the tshirt with FIASOI t-shirt out in public
people sometimes would look at it and say what is that about
and I would say its my Philosophy that I live buy > it stands for
Forever In A State Of Improvement

and I would just tell people I have decided to live my life that way that I will
constantly be living my life in such a way where I will be forever trying to better myself
and improve upon myself.

So get yourself in the FIASOI mode
get your self in a State of Improvement mode
when you do that you wont need to feel bad or think about hurting yourself
Youll see progress in your life and youll be happy to see positive changes in your life
and better things will come your way

Good luck