I am sorry this is happening to you.
This is a personal opinion and not a professional answer. I will have to make assumptions on what you have written.
“…I’m a really nice person…”
The nice person persona as a survival strategy
Depending on how you were raised, you possibly feel obliged to be nice. So if you have authoritarian parents, come from a religious background or a background with a strict doctrine or you have something physical/psychological about you that differentiates you from others to the point that you are in a powerless position in relation to your peers. e.g. if you are a male you are smaller than others, if you are female you are not as attractive to others, if you are slow, if you are LGBT, if you are of different ethnic origin to others in your neighbourhood, or If you have a disability. It may not always be the case but usually if someone is an outsider (of some sort) to the power group than this person has to work extra hard to be excepted. This includes being on the receiving end of questionable actions and attitudes.
If this is the case then you would have had to employ the approachable attitude of ‘I am a very nice person’ persona in order to survive your environment. You are not a bad person and you have done nothing wrong.
‘…Why are people leaving me and forgetting about me…’
The people you are hanging around with may not be so nice
Possibly because as much as they like to put across that they are nice, they are not. You can run into a situation where people will let you hang around them as some sort of public service activity, e.g. we don’t think that much of him/her but out of the goodness of our hearts we will let them hang around and tolerate them.
Understand that this attitude can be used against anyone regardless of who they are and how dynamic they are as a person. You can get groups out there that really are poisonous, small town mentality so to speak. You remember the tale of the ‘Ugly Duckling’. Well that Swan was surrounded by ducks and other animals that exhibit the ‘small town mentality’.
Finding the real you.
What can help is having a bit more faith in who you are. If you have been in an authoritarian environment you may be disconnected to the real person that you are.
I talk about this and the concept of feeling invisible in the below answer. If you look at the answer it will have links to articles that you could check out. Gabrielle Brendale's answer to How can I build self-esteem as a teenager? You may want to get a journal/diary and just record how you feel each day. this is personal stuff and not for the eyes of another, just record honestly what you think, don’t censor it. Be as nasty and confrontational as you like. This tunes you into your ideas and values.
Be aware that you could also be in an environment where someone is gaslighting you or crazymaking. Crazy Making, How to Handle a Crazymaker, I have just done a quick scan of these. See if they help. 10 Signs You Are a Victim of Gaslighting - David Avocado Wolfe, This one is on gaslighting, see if it helps.
I also talk about smiling too much in the next answer. This also has links to articles. Gabrielle Brendale's answer to Should I stop smiling too much when taking or listening to people?
Good social skills include having an awareness of what is happening around you. If you are too busy concentrating on impressing people you may be missing important subtle information.
People-Pleasing: The Hidden Dangers of Always Being “Too Nice” ⋆ LonerWolf This is a article about people pleasing. You may also want to look at neediness, How To Eliminate Neediness - 5 Proven Tips - Daniel Blake He is on youtube, more for about trying to hook up with girls but it has some really good ideas you can employ for day to day interactions.
Boundaries, this is a site on boundary setting. Put these into place and also ensure that you respect other people’s boundaries. Oh, and if the people who hang around you are use to you not having good boundaries and taking advantage of you, then you will probably cop grief for setting boundaries, just warning you.
You may also want to brush up on your social skills and interactions. Social Skills Picture Book for High School and Beyond: Jed Baker: 9781932565355: Amazon.com: Books This is a pretty good book - not only for people with autism, anyone can use it. See if your library has it.
Below is about Transactional Analysis - This is all about communication information and how we communicate with others. This is from another of my answers (I will put the link in as it also has a few links to sites.
‘…Transactional Analysis 1: ego states & basic transactions - TheraminTrees
He has three videos on Transactional Analysis - they are pretty good and break down the information so that it is easy to understand. You might not agree with everything in his other stuff but the TA stuff is pretty good…’
This is also from the same answer. It is about situations where you are being taken advantage of.
‘…Tips and tricks - I know I use to fall for the following. Person says to me. “What are you doing on Tuesday lunch time?” Me “Nothing” Person “OH, great, I going to lunch with the others in the office and we need someone to look after reception”. or something to that effect. If anyone says, ‘What are you doing (date) (time)?” without disclosing what they want from you, always say that you are busy. If it is an invitation you can always say yes later…’
From Gabrielle Brendale's answer to How do I stop being a target for people that manipulate or like to bully?
These next two sites are about how others can manipulate you. Don’t read up on them to know how to manipulate others. Rather, read up on them to know when it is happening to you.
"Get Anyone to Do Anything and Never Feel Powerless Again"
Try and break out of your really nice person persona. Unless you are breaking the law, hurting another or about to do an action that affects others, then you are really not required to justify you actions to others…well not unless they are the police etc. But you get what I mean. Good luck.