Foster thought that his side had done enough to claim all three points when Thievy Bifouma put them in front in the fourth minute of stoppage time, but Mats Moller Daehli hit back for Cardiff straight away to leave the Baggies without in win in seven home games.
"It was horrible last weekend," Foster told reporters ahead of today's trip to Norwich City. "It was probably the saddest I've ever been after a football game. It was absolute elation turned to pure sadness.
"But we've got to look at the positives and the fact we actually played really, really well. First half we blew them away. Second half there was a period where the lads probably had to get their wind back, but we went again, then probably for the last 15 minutes we were the top team.
"Overall, we were fairly unlucky. We need to take those positives into the Norwich game."
West Brom begin this weekend's round of fixtures in 17th place in the Premier League table, just three points clear of the relegation zone.