Milan came from a goal behind to win 2-1.
"It's difficult to get over this," Italy international Motta told the official Inter website.
"We made a few mistakes too and it's a game we could've controlled better in the second half, especially as Milan didn't do much.
"We kept the defence tight in the first half and all of us knew exactly what to do. Then we fell back a little, but it's not as if the Rossoneri pushed us particularly hard.
"Their first goal came from a strange situation and the second was a long ball."