I’m so sorry you are in this painful situation. Sadly, many adults don’t understand that when they make life decisions, it can have enormous impact on their kids. My guess is that your dad’s decision doesn’t really have to do much with you as a person — although of course you take it personally since it’s affecting your life. Your dad has been unhappy and he is grabbing at a new start to solve all his problems. It won’t work, of course. He can’t erase his past. His life with his new girlfriend and her kids is likely to be as complicated and challenging as his life has been before. For one thing, like you, her kids may not be welcoming the change in their lives.
Unless and until your dad figures out that the “grass isn’t really greener” elsewhere, there is little you can do to change his mind. Hopefully, at some point he will come to his senses and regret this decision. But even if he doesn’t, his rejection doesn’t mean that you’re unloveable or that you aren’t worth someone’s attention. You are. Make friends with your grandparents.You and they are in the same boat — out of control of your dad’s behavior. Reject your dad’s rejection and find other adults who appreciate you and who are willing to help you on your way. Set some goals for yourself. It’s impressive you have brought up your grades. Keep it up. That’s going to help you make the life you want someday.
I understand that the situation hurts and is terribly unfair. But you don’t have to let it ruin your life.
I wish you well.