Struggling to deal with someone changing?

When I was a kid, maybe about 12 or 13, whenever my mom and (at the time) step dad would go out in the car I’d sit in the back seat and he would be driving and randomly reach behind to me and dig his fingers into my knees and laugh. I shrugged it off as him to being playing in a rough way bc he was like that but the more I look back at his other behaviours it makes me think. He wasn’t gentle when he did it, he’d really dig is fingers quite hard bc he know it would get a reaction from me. When he first met my mom he was great, treated me like his own kid (he had his own kids as well) and he spent a lot of time at our house. But then he would start promising me things and never follow through with them, he started to drink quite a lot, he and my mom had a baby when I was 15 and when my mom was busy and asked him for help with the baby he would just pass the baby to me and drink instead. I remember one time when my brother was a newborn my mom asked my step dad to feed him his bottle so she could get his clothes and crib ready for bed…he fed him for about a minute before handing him to me and telling me to feed him so he could sit in the kitchen with his vape and alcohol. He eventually left us all together and never visited my brother after the age of 2. I just feel confused and hurt because when he first came into my life he was like a true dad to me and I loved him, things were great, he was engaged to my mom and then he just changed.
Struggling to deal with someone changing?
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