I think "про" implies certain narrative or a certain concrete thought.
Я думала о тебе - I thought about you
Я думала про тебя, что ты алкоголик - I thought about you that you're a drunkard. ("о" is bad here)
Мне рассказывали о тебе - I was told about you
Мне рассказывали про тебя ужасные вещи - I was told bad things about you.
I would also say that whеn referring to a person not introduced before, you better use "про".
Девушки рассказывали про своих молодых людей - The girls were telling about their boyfriends (this kind of implies that you did not know about them before or at least they are not familiar to you)
Девушки рассказывали о моем молодом человеке - The girls were telling about my boyfriend
but when there is narrative
Девушки рассказывали плохие вещи про моего молодого человека - The girls were saying bad things about my boyfriend
Девушки рассказывали про плохое поведение моего молодого человека - The girls were telling about the bad behavior of my boyfriend
In general if you learn something new and concrete, you usually use "про".
If you love a man and just want to say that you are thinking about him and being with him, you say
Я думала о тебе - I thought about you
But if you thought what to do with him (or with your relations) then you would use "про"
Я думала про тебя... Я думаю, нам лучше расстаться. - I thought about what to do with you (with our relations). I think we better part ways.
"о" implies intimate thoughts about a person while "про" implies some distance.
Я думал про тебя. Я думаю, мы найдем применение твоим способностям даже не смотря на то, что других сотрудников уволят. - I thought about [what to do with] you. I think we can make use of your talents, even though other coworkers are going to be fired.
Я думал о тебе - тебе будет нелегко после смерти матери. - I thought about you. I think it will be difficult for you after the death of your mother.