As a novice player, I see this as my worst leak. Not folding the second best hand. I agree with folks saying a call on the river was a better move but hindsight is 20/20. You mentioned you considered the villain had the ace, somtimes you gotta listen to what the chips are saying. A raise on the flop could be a bluff but if v keeps barreling they probably have something. Perhaps, you could have reraised v on the flop, repping 3oak and they might have folded their 4 flush with Ad. Big hypothetical, although, if v called the reraise and fired agian on the turn it would be a little more obvious they had the nuts.