1.) If Trump knew he was innocent of any crime, collusion or obstruction, he wouldn't have attempted to fire Mueller. He'd let the investigation run it's course and prove to the world he was innocent.
2.) He didn't try to fire Mueller until he found out that Mueller had included obstruction, vis-a-vis his firing of Comey, into the scope of his investigation. So now, even if Trump was innocent of colluding with Russia during the campaign, it doesn't matter anyway because he's now obstructing an investigation into obstruction.
Trump fucked himself either way.