Neirin, I'm so very sorry for what you've been through. You're definitely not "complaining about nothing." This is clearly something that's been bothering you for a while, and your feelings are totally justified. Your post actually brought me to tears - I am really sorry for the position she's forced you into.
About whether it was abuse or not, it's up to you to determine how you frame it, but by law, what she did was without a doubt sexual abuse. You were not old enough to consent to sex with an adult. There are many good reasons for this of course - adults are in a position of power and responsibility. Your father's girlfriend took advantage of that, and groomed
you until you thought that sex was something you wanted. It was never what you really wanted though; your words make that pretty clear. "I don't know when she started doing it." - this is a very important point - remember that you
did not start this, she did. "[She] manipulated me into thinking I wanted it" - this is another very clear indication of what was happening. The word MANIPULATED reads very loud for me in your post, not just the one instance of the word itself, but it echos throughout the rest of your message.
What happened to you was not your fault! Please don't blame yourself. She made you do these things. She knew that she was doing something that was wrong, and as an adult, she had a responsibility to know better. What happened was her fault entirely, and not yours.
It's clear that you were under significant emotional strain while she was grooming and abusing you. It's also clear that you were very conflicted about what was happening when you were younger, and that confusion about how to think about what happened is still very much alive in mind head today. You can't quite figure out if it was something you wanted, or didn't want. I'm not sure if I need to say this or not, but physical arousal, either now or when you were younger, is not an indicator of consent, genuine desire, or affection. It certainly does not mean that the mind is/was free of internal conflict on sex in such a complicated situation. Pretending to be asleep when she'd come into your room uninvited makes it pretty clear that it wasn't something you wanted. I'm so sorry for all that she did. You didn't deserve to be treated that way.
Why do you feel so dirty and disgusting now? How do you feel when you think of what she did to you? Have you ever talked to a therapist about what happened?