Why are people so careless?

To begin no you are not a mistake and yes you belong on this earth, you maybe depressed, which is a serious thing but also treatable. Its also very admirable that you are seeking help for this which can be scary or difficult.
From this post and a few others you seem kind of young, these means your peer group is probably also rather young. The difficulty here is that young people rarely have the emotional maturity to deal with difficult social issues and so for your friends its likely they are not intentionally bullying you but more they don't know how to help, don't know how to identify with a grieving friend and maybe even can not understand your feelings or moods because they themselves are not going through it.
Even with adults, being a good supportive friend to someone who is depressed can be difficult and sometimes they pull away because of those sad feelings. They feel that its a downer and may not realize that they are in fact making it worse.
Have you spoken to a counselor about your step fathers death or these social issues you are facing? This may be your best option. See if your school has a counselor available and meet with them to see if they are someone who could help you, my highschool offered the option of weekly appointments for difficult situations and even had a group session for grief and loss. If that is available I encourage you to take advantage of it.
If in the end your friends did pull away because of an en ability to understand your situation then maybe you could try and talk to some of them at some point and rekindle your friendship.

I wish you well.

I think it's important to to know that your value is not depending on the people in your environment. You seem to doubt a lot about yourself, but I can guarantee that you are no mistake. Nobody is a mistake. The first step is to acccept yourself just the way you are. And trust me, you are just fine the way you are and if the people around you can't appreciate that, then they don't deserve any credit from you. Sometimes we cannot change the set of people we meet regularly. But with the passing of the years you can actively change your environment and look for new friends and people who care about you. But the first step is to acknowledge and accept yourself the way you are. Staying positive will affect people in a good way and they start looking up to you.

You don't need people to tell you you're worth it because you already are, trust me. And maybe it's a good thing your ex-best friend is out of the picture because now you know he's not a good friend. You're a wonderful person. Hang in there

Everyone has a purpose on this planet, and some people just don't know it yet. Sometimes friends can be a little out there, and these days there are stupid people that don't pay attention to someone who really needs it. That's why there's the joy of the internet, the social media. There are always people online who would gladly give you some good words to send you some virtual hugs. When it comes to me, I always have my online friends, and my real life friends. Either way I always have someone there for me no matter what happens.So just hang in there, keep strong and strive to find that main purpose you have in life. Keep people close to you and find the people in life that will make you happy.