Your parents went their separate ways when you were very young, and because of living near each other it had been easy for you to see your dad regularly while living at home with your mum.
You have been brought up with “your dad is your dad” and even though you have tried to see someone important to you in him, tried to find some good things about him, you have always struggled to get close to him.
Today you are a beautiful young lady but you are still suffering from this father that is not and has never been a father to you, never been what you wanted him to be, what you needed him to be: take you on his lap and read you stories, celebrate your birthdays, be proud of your school results and your uni achievements…
It’s true that all of that is and has been painful ( your dad being bipolar, he has never had it easy, and neither has it been for his present partner. But that consideration is beside the point here)
So hear what I have to say to you: Now that you are an adult, you can forgive your dad his inadequacies as a father and let him live his life. Wish him good luck and be generous. Let him go.
Then find another man, someone who could be your dad, someone you really like, and someone who likes and respects you for who you are: a beautiful young lady he would have loved to have as a daughter.
Talk to him, take your time, then invite all your friends and people you love and have a big adoption party in which you declare him your dad and he declares you his daughter.
It will be symbolic and profound.
He will always be there for you and you for him. He will be the one to carry your children on his shoulders and make them laugh with silly jokes like only a grand-dad can. You’ll visit him regularly and you’ll worry about his health and his memory declining.
You don’t have to suffer anymore.