Hi there waterhyacinth, firstly let me start by echoing Sam, you really do deserve so much better than this, and whilst it seems quite confronting an idea at the moment, you will be able to get through this.
With the idea of being "alone", whilst it can seem like that would be the case, abusive and emotionally manipulative partners are really good at trying to make their partners feel that their relationship is the only meaningful one in their lives, and that they would be "alone" and "helpless" without them. However I can already hear from you that you are not alone, you have meaningful relationships in your life, with friends who care about your well-being, and a sister who does too! A good support network needs more than one person, and a romantic and/or sexual relationship is not inherently superior to other kinds of relationships, and certainly you will not be alone just because you don't currently have a romantic partner. You can read a bit more about why friendships are so important (and awesome), here: Why Does Friendship Matter to a Sex Ed Organization?
With regards to your concerns about breaking up and your boyfriend not accepting it, even in this case, you are not in a relationship with him. Planning for potential issues and outcomes like you describe is called safety planning, you can do this before you do the breaking up, and it can help to do it with people you trust, like your close friend, and your sister, and write down what you're going to do if certain things come up. Is your sister older than you? Having an adult you can trust in this experience can really help, acknowledging that you are almost an adult yourself, someone with the autonomy, resources of an adult on your side can really help.
I will put some more links to safety plans below, but to start with some ideas on the issues you mentioned:
If he tries to keep contacting you, you have every right to block him on every form of social media etc. you have. If you can, you can get a new SIM card for your phone and a new number. If people who you don't feel comfortable sharing the reason with ask (most won't), just say you got a better deal with a different carrier.
If he will turn up places, make plans to be there with your sibling, friends, and get really busy hanging out with them, even if it's busy playing games or video chats with them if you're not going out much at the moment because of the pandemic. This will also help you focus on the great people in your life, rather than him.
I hope this helps, as always, please feel free to share your thoughts and feelings about this with us here.