Turgon's Folly
 

Why did Turgon became proud and refused the aid of Ulmo?
Was it because he was loath to abandon anything he had set his mind to do?
Why is it that he trusted Turgon in the beginning and not in the end?
What happened to him?

He loved too well the works of his hands and the devices of his heart. He also misplaced his trust in the secrecy of his stronghold, even though Ulmo, who would know better than he, warned him not to.

It was probably more a matter of not wanting to abandon all the neat stuff he had built, combined with not wanting to give up his kingdom.

I'm not sure I understand who you mean. If you mean Ulmo, I would say that it might not have been a matter of trust in the first place. Ulmo was trying to give Turgon good advice so that he would be able to endure through the Morgoth Wars and survive at the end.

He either died fighting in front of his tower, or died when the tower collapsed on him. One of the two, can't remember which.

Had Ulmo not trusted Turgon early on, the fate of Middle-earth could have been much worse. Although Turgon eventually forsook the counsel of Ulmo, he did manage to stay hidden long enough for Earendil the mariner to be born. So Ulmo's trust was not completely misguided. He knew in his wisdom that the Hope of the Edain and Atani would come from Turgon's hidden city. How much he knew, well...

I'll consider the other interpretation of Maedhros' question, that he meant to ask why Turgon trusted Ulmo in the beginning, when he was guided to that hidden lake bed, but not at the end, when he was warned that the fortress city would eventually be breached.

What happened in between? Turgon became a founder, a builder, and a creator. He was Noldor, and the act of building his beautiful Gondolin awoke the fire of Feanor in his heart. He made the same mistake of placing his own sub-creation, glorious as it was, over all else, including the love he knew Ulmo bore him and his, and the utmost wellbeing of his people and kin. He risked them for a city, for architecture. He was honestly mistaken about the best course and the soundness of Ulmo's intelligence, but he also weighed the value of his city too heavily when he should have thought only of his people within it.

However, Turgon's mistake was much less serious than Feanor's and his sons'. It certainly led to a fatal error in his strategy for the defense of his people, but I don't think it rose to Feanor's level, selfishly denying the possibility that others also loved THEIR creations with a love as true as his own, including Eru and Arda, but also other sub-creators such as the Teleri with their white ships and Yavenna with the possibility of her healing the two trees.

Turgon's was an ironic choice because remaining in Gondolin was to all appearences the prudent, cautious conservative thing to do, but given Ulmo's advice, it was deeply reckless. I have a lot of sympathy for Turgon's mistaken strategy, though-- he was facing Morgoth and all his horrible army, who wouldn't want to hide in the safe, good place they had found and made so beautiful. I wonder if Tolkien was thinking of the French generals' disasterous reliance on the supremely well fortified Maginot Line in WWI when he devised Turgon's strategy, which was so prudently reckless.

Truly, this is the kind of answer I was looking for. Great answer.
I agree, but Feanor was a unique elf in the way that his works could rival those of the Valar themselves.
I would say the mistake was Feanors and not of his sons. They didn't create anything like their father. They were just following him.