Well first thing I would do is reraise the raiser to $40 total when he raised to $10, but since you didn't and were in this situation, I think you'd find yourself in a tough spot

I'd count my outs, 9 more diamonds to a nut flush, plus I'd feel confident that any King or Ace would give me top pair and the pot, so I'd give myself an additional 3 outs for the remaining Kings and 3 outs for the remaining Aces, for a total of 15 outs, the rule of 2 and 4 and you have a 60% chance after the flop and 30% chance still after the turn if it doesn't not improve your hand. I'd also factor in another .5% in case the turn and river are an 8 and a 5 not of diamonds to give you a split pot. I would with that information, raise probably all in and hope he folds, but not hate myself if he calls