The Robins clinched a play-off spot with a 3-0 victory over Westley's men, who were forced to play with 10 men for 50 minutes following Dani Lopez's dismissal
Westley said: "We have been shown exactly what the play-off sides have got what we haven't in terms of organisation, physicality, mentality and attitude to winning.
"There are all sorts of gaps between us and we face a long hard pre-season to put in place the things that aren't in place.
"All the goals were preventable, two certainly were the result of the ball bobbling about in our penalty area.
"You don't see too many Stevenage sides letting goals in like that, our fans are used to seeing players put their bodies in the way and that's something we need to instil in the close season."
Lopez received his red card five minutes before half-time. Referee Gary Sutton was left with little option but to apply the letter of the law as the young striker raised his hands and appeared to slap Swindon skipper Alan McCormack.
Westley added: "I didn't see enough of the sending off incident to make a comment. People who did see it say that Dani was hit first and then tapped the fellow around the face.
"I'm not sure what level of aggression there was but the kid is in the dressing room crying his eyes out because he feels the decision was harsh. It certainly was if he was hit first."