The 20-year-old striker had possession deep into stoppage time but opted against trying to run the clock down - a decision that proved costly when he lost the ball and the visitors broke to level it up.
It left Mel frustrated as two crucial points in the battle for Premier League survival slipped through his grasp.
"With both (teams) looking for the same things and both teams looking for the win I think frankly today West Brom were better," he said.
"A wrong decision was made today, we were 3-2 up with just 30 seconds remaining and that should have been the end of the game.
"Saido's a young player and went in search of a fourth goal instead of taking the ball in the corner. But he will learn from this."
The hosts began the game brightly at The Hawthorns and found themselves 2-0 up after just nine minutes courtesy of Morgan Amalfitano and Graham Dorrans.
In what was an exhilarating affair, Cardiff drew level thanks to goals in either half from Jordon Mutch and Steven Caulker.
Yet still the drama continued, with Thievy Bifouma scoring in injury time only for Mats Daehli to capitalise on Berahino's error in judgement to make it 3-3.
A victory would have moved Mel's men six points clear of their visitors and, with Crystal Palace having secured a shock 1-0 win over Chelsea, the Spaniard confessed it had been a gut-wrenching day for West Brom.
"(We are feeling) quite bad really and the spirits in the changing room at the minute are quite bad," he added.
"The fans will see that we played a good game and the result should have been a better one really.
"We really needed the three points and we really had the opportunity to get those points. The fact we have drawn is down to our own errors."
