I (23F) never told this side of the story before. My family knows what happened but doesn't know why.
When I was 15 my dad made a new friend who got really close to us and started hanging around all the time. We all loved him, he was like the adopted uncle of the family. He always showed more interest in me than my other siblings and went out of his way to make me feel special. He also gave me a lot of advice and told me he loved and cared about me every day. By time I grew attached to him.
I started feeling jealous whenever he showed any attention to my other siblings or other "kids" around my age. I claimed him as my best friend and also felt a deeper attachment to him, but it was NEVER romantic. He started acting weirder and weirder. 
He would tell me he's looking forward for me to turn 18 so he can hug me. He would say he wishes he was my age so he could marry me. I told him it was weird of him to say that, and he ignored me for a day so I apologized and started to ignore those things because I felt like I needed him. I was always a qyiet person who never told anyone anything personal before. The more I told him the more attached I grew to him. For a few years we lost contact and later He reappeared, engaged to a beautiful woman. I was happy I could finally be his friend without that "weird" feeling that he wants more from me. Whew child was I wrong...
We started communicating again and I told him EVERYTHING. Our conversations revolved around me. He would always praise me and make me feel like the most special person in the world. In exchange I got attached to him. I slowly began to idealize him and look at him as my "super hero". I secretly started wishing he was my dad. I wished he would just once tell me he wished I was his daughter too - but he never did. I kept excusing his behavior. He would buy me gifts and lie to his wife because of me. He would text me all day long and get worried if I didn't answer in one minute. After a while he would touch me and give me tight hugs and kisses which I was too afraid to reject. I felt like if I rejected anything he did to me he would abadon me... So I just let him do those things. I would simply lie to myself they were not happening.
Suddenly I was spiraling out of control. I cut everyone off who thought anything bad about him or our "friendship" so I was left with nobody but him. I believed that nobody loves me but him.
My whole world revolved around him. I wouldn't do anything all day but sit in my room texting him. I started changing myself letting go of more boundaries, so he wouldn't abandon me. But I was messing up. I got upset at everything he did. If he gave a hint of any romantic feelings towards me I'd freak out but still I end up apologizing so he wouldn't leave. 
I was becoming emotionally unstable and started having major breakdowns. I really needed a friend, but when I texted him he would just say weird stuff to me like "I wish I could feel your skin" or "I wish I could kiss your beautiful hair. I felt objectified and blamed myself for the way he was acting. I cut my waist length hair, stopped using makeup and started wearing lose clothes because I didn't want him to look at me as a woman... Every day I'd get upset at him. Suddenly he started acting different. He started ignoring me and telling me that what we're doing is wrong. Every day for months I'd break down and cry in my room, on public transportation, at work. I could just FEEL he was about to abandon me.
One day I got a text from his wife. She told me she read everything I sent to him. This is the first time I thought about her and what she must think. 
The man deleted everything he  sent to me but kept my desperate "break down" messages. It seemed like I was in love with him and he didn't want anything from me. The wife sent screenshots to other people and I never felt more embarrassed. I was still attached to him so I didn't tell her what he did. I regret that to this day.
I did formally explain to her that I never felt anything romantic towards her husband. She was understanding but that's all about it.
After this day the man blocked me everywhere. I felt so abandoned and cheated but looking back, I'm grateful he cut me off and stopped ruining my life.
After this incident I changed so much and I still miss the person I was before him. I'm glad it pushed me to realize I have a big problem inside me but I don't think it was necessary. I just KNOW someone who's not bpd wouldn't have let his happen because he did try this with others too, still I'm the only one who ended up like this.