boy advice.

okay so i’ve never had a relationship longer then 7 months and that was when i was in 5th grade and i’m currently in 9th. since 5th grade i’ve had about 4 boyfriends and 1 girlfriend (i’m bisexual) since then but it never seems to work out. with every single guy (nothing really went wrong with the girl) i start talking to them for a really long time because i don’t like to rush into relationships, then they ask me out or i ask them out. after that it’s good but then after about 3 or 4 weeks they seem to show a new side of them. like for example my last boyfriend was super sweet and understanding in the beginning but then all that went away and he started being manipulative and controlling and just weird and all about sex which i’ve never had before and my body so i broke up with him. i don’t know how to choose good guys from bad. is there any advice anyone could give me on some hints or tips or literally just anything you have to say about this??? it would be reallllly helpful.