McCarthy's side built up a 2-0 lead thanks to first-half goals from DJ Campbell and Daryl Murphy.
Charlton hit back with a penalty from Danny Haynes but Ipswich hung on to claim the victory at The Valley.
"The self-belief comes with winning games," McCarthy said. "It doesn't come from me, it comes from within.
"They are professional players and it helps when you are winning, or at least putting in decent performances.
"It gets a bit flaky at times and you wonder if you are going to be able to hang on. But they did and I'm pleased with them."
McCarthy was pleased to see the strike partnership pay off with both players on the score-sheet and said: "The two penalties opened the game up. We missed ours but it gave us an impetus to go on.
"We got good quality goals. DJ persisted after the miss and took his very well.
"They put us under pressure in the second half. They have two good centre-forwards who gave us a lot of problems but we persisted and held on."