Hi and welcome Fritz;
I'm sorry to hear you've been hurt by this situation. Hearing about your Dad's other daughter would've been quite a shock. Talking about it on here is a really healthy way to nut out your deep feelings; it takes courage to reach out for help so well done. 👍
Fortunately for you, (in my opinion) you don't have PTSD, however unresolved issues still linger to torment you by the sounds of it.
Knowing there's someone else out there who's inside your dad's and bf's heads and can't be taken away, must make you feel pretty helpless. No matter what you do, say or think, you can't change things.
Both men in your life are carrying love for someone else and you don't know what to do about it yet. This double-header would be difficult to face at your young age, so I'm not surprised it's swirling around your mind like a tornado.
Talking with dad about your feelings of not being his (only) little girl anymore might help both of you understand where to go next. From my perspective this is probably the cause of your suffering.
Asking him to reassure you that he still loves you as he always did might help you feel better. After-all, he's known about her all this time and still loved you. There's nothing different now except he's disclosed this information; you're still his number one daughter.
With your bf, things are a bit more complicated. If your quarrels have been about this fantasy girl, then it's not going to go away anytime soon. You need to decide what's best for you and your future; stay or leave.
It's not that simple I know, but it must be playing on your mind, and as you're so young and inexperienced with this type of conflict, your confidence and trust in 'yourself' is probably what's causing issues.
You've never had to confront such adult concepts before; this is a learning opportunity and like all of us, growth can only be attained through trial and error. It's hard, but it's a fact of life.
You can approach these issues as a confused and wounded little girl, or treat each as an adult who has the rest of your life to learn so much more about problem solving and protecting yourself from emotional pain.
I'll check in each day to see if you've responded. I'd like to discuss trust next time if that's ok.