WHY IS MY DAD CHEATING ON MY MOM??!?

[mirabelle]

im a 13 year old girl living with my 2 married parents

when my dad was in the shower i looked on his phone and there were loads of messages(on his facebook)he sent to women like"love u babes xxx" and "i want the best for you xx" and "i will always find a place for you in my heart xxx".i was so shock i couldnt read more.i know it was wrong of me to look at his messages but....

when i was little my dad cheated on my mum and they separeted.i was really horrible because my mum went into a deopression(shes bipolar)and she had to go to hospital.

after a year they got back together and now,im 13,and i find out he doing it again!

i really dont wanna tell my mum and i dont have any family in this country!

WHAT SHOULD I DO??!please help

P.s.i took pics of the messagrs(they're in my secret folder on my phone)so i have proof.

no answers like "stop snooping around your dad's phone!" or "this is an adult thing"

also its not family or good friend because i looked at her profile and its a woman in her twenties that live in the same town as us!(weve only been living her 3 years!)

i know i shouldnt have looked at the messages but i did...

also im not confident enough to tell my mum because i love her so much i dont want her to feel hurt!!!

btw mat i point out he is a 50 year old methodist(christian priest)!!!