But the U’s boss spoke of his admiration for his players after they booked their spot in the Wembley final next week.
Kenny Jackett's side’s promotion push fell short at the play-off semi-final stage for successive seasons.
The Blues were held to a 2-2 draw against Oxford over the two legs, but lost 5-4 on penalties.
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In total, the two sides fought out a stalemate on four occasions this campaign, having also been deadlocked a 1-1 league meeting in November and a2-2 Leasing.com Trophy clash a month earlier.
Robinson admitted the latest encounter wasn’t the easiest on the eye, with goalscoring chances few and far between.
But he hailed his Oxford troops as they prepare to face Wycombe in the final at Wembley on Monday, July 13.
Robinson said: ‘I'm just full of admiration for the players. We're in the final – what more can you say?
‘When you think that I've had to sell my two best players in January and we came into this game with three or four struggling with injuries, it's incredible.
‘I had asked a few people in the game who have been in big penalty shootouts for their advice, and they passed that on, and through our sports psychologist too who was telling the players to be composed and just believe in themselves.
‘I'm just gutted for our fans who couldn't be here to celebrate it, and I'm gutted there wasn't a full house in here to see this.
‘But for Oxford United to be in the final is unbelievable. Jamie Mackie's in tears in the dressing room and he got me going too!
‘I'm immediately proud of the appetite and desire of my players.
‘As far as us and Pompey are concerned, we both finished on 60 points and we've played out three 1-1 draws in our meetings this season. That shows how evenly-matched we are.
‘It’s two wonderful teams and it wasn’t a great spectacle for obvious reasons but we both know what we’re good at and we stop each other in a very clear way.
‘I have to say we're a little bit off it without our fans, they make a big difference to us here.’