I am sure this was a total shock. I'm sorry about that. Try not to be devastated. And the man who raised you IS your father. Not some guy who put it in your mom a few times. He really doesn't matter other than basic DNA. Your dad raised you, loved you, etc. Your mom probably did what she could to protect you and herself and provide a two parent, loving, stable home. I'm sure your parents weren't perfect but tried. Like we all do.
So, don't make your mom suffer for this or dishonor your dad by acting like some other stranger is the dad you need to get to know . . . and he isn't anymore. HE is the man who has cared for and loved you.
Let us know how it goes. peace