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 telling bad things about my boyfriend

Девушки рассказывали про плохое поведение моего молодого человека - The girls were telling about 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 about him and being with him, you say

Я думала о тебе - I thought about you

But if you thought what to with him (or with your relations) then you would use "про"

Я думала про тебя... Я думаю, нам лучше расстаться. - I thought what to do with you (with our relations). I think we better to depart.

"о" implyes intimete thoughts about a person while "про" implyes 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 stuff is 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.