EXACT same thing happened to me when I first got my Rivendell mixte. I'll save you the trouble and give you the solution
I was falling forward on the saddle and thought I must need to slide the seat closer to the bars, but even after that, I kept having the same issue. Could not understand it. I tilted the nose of my Brooks up more, which only resulted in more discomfort and STILL sliding forward. I then blamed my posture. Figured I'd have to train myself to hold my body back on the seat. Them I came across something Grant Petersen (of Rivendell) said. He said most folks he knew slammed their saddle all the way BACK and that was the key to comfort. It was counterintuitive, but I finally tried it. Bingo! Perfect comfort. Seat is right under those sit bones, no unpleasant friction in any special spots, and NO MORE FALLING FORWARD. Try it, you'll see!
Wherever I go, there I am!
2012 Rivendell Betty Foy