husband cheating. have three kids together.

my husband and i have 3 kids together and recently he hasn’t been acting the same and i figure he has just been stressed about work. last night while he was in the shower his brother texted him and i was going to respond for him and he had a conversation from a number i did not know. i started reading it and this is what it said

He said I am gonna take out this week.

her: You are?

him: Yes how about Tuesday?

No response.

her:

How did your fishing go today.

he Sends like 4 pics tells her all about the fish Then he says I love you babe. and says he wants to marry her

i am heartbroken. what do i do? our kids are all under the age of 8. someone please help