DAFUQ IS MY DAD DOING???!?

[mirabelle]

im a 13 year old girl.i have no siblings or close family.i live with my 2 married parents.
im not gonna confront my dad or tell my mum(shes bipolar).

on my dad's phone(blackberry bbm):
"thats a sexy profile pic,makes me wanna hold u.love u babes miss u too xx"

"awww thanks sweetie love u too xx"

"you will always have a place in my heart and i will always make time for u"

in another convo on his facebook with the same woman:
"cant wait to see u hunny xx"

"miss u so much x"

in another convo:
"just got out the shower"
"wish i was there to dry u xxx"

its definativly not my mum or family.i checked her profile:
i have her phone num,home address,name.
she lives an hour away from us.shes in her 30ies.
my dad is 51 years old and is a protestant preacher!

who could i speak too?what should i do?

btw i took snap shoots so i have proof.i know it was wrong of me to look in his messages.:-(

IM A NOT GONNA SPEAK TO MY MUM OR DAD ABOUT THIS

also my dad cheated on her before when i was 6.she became very ill and deppressed so she ended up in hospital.
after a year she gave him a second chance.
i dont want this to happen again!

please HELP D: