I would have a hard time folding this. I would really need to see how villain has been playing his hands, however. Some stats, whether he likes to push. You need a little more information here. Also, I think I said this before, but I would much rather see a pot sized bet on the turn than a weak 3/4th-ish one. But, if you know yourself and are going to be squeamish with a third diamond, then might as well be a little more conservative with the pot from the get-go.

Also, as to the possibilities of his hand, assuming your opponent is logical and has some respect for your game, you can narrow his hand down, at least beyond two pair.

One thing I don't advocate is overbetting on the turn to justify a call. Look - if you think that your opponent is on the flush and/or have no reads on him to believe otherwise, then putting more money in just to lose more money is a very unprofitable proposition.

When all is said and done, results would be appreciated.