Preflop raise is too big, as you know; I think around 100 is good. Get all in on the flop every time here. Most of the time you will be shown a lower pocket pair, A6, or a draw. Most lower buy in players love to slowplay bigger hands, you will very rarely be shown a hand that beats you here. You got unlucky to happen to run into a big hand this time, but don't be results oriented. Getting all in on the flop in this situation is definitely +EV.