I'll turn the tables here:
1) Why should Iran come to the negotiating table if the previous deal was scuttled so quickly? What even makes American treaties reliable any more? To put it another way, why should they even trust any treaty with the West? Why is Iran asked to turn the other cheek, vs America asked to provide concessions for overturning a treaty, or put hard checks on their client state, Israel.
2) If you have more to give up, wouldn't you have a stronger bargaining position? There's not much the West can do at this point in terms of tightening sanctions, and they, with the exception of Israel, have no interest in another long war in the Middle East.
3) Why should Iran not be able to develop a nuclear deterrent? It is surrounded by American bases in Iraq and Afghanistan, has Saudi and the Gulf monarchies against it close by, and is an enemy of Israel, which is known to have dozens to hundreds of nuclear warheads. Clearly, given MAD, launching nuclear weapons offensively is untenable. So, a nuclear deterrent might protect Iran from foreign attack. A deterrent means the US or its client states aren't allowed to unilaterally bomb Iran any more, which is the right they are currently claiming.
I'm simply asking you to consider their view, without considering them irrational or chaotic or evil a priori. Certainly, a great deal of prejudice, jingoistic nationalism, sectarian religiosity, and macho posturing are at play on their end, as well. I can say it's similar on the end of Israel, Saudi Arabia, and the US, as well. A more balanced view, understanding the motivations of all sides, may help arrive at an actual solution to this awful standoff, where innocents bear the brunt.
I sincerely wish all the countries of this region, and across the world, disarmed themselves of nuclear weapons. It's too easy for disaster to occur, accidentally or at will, and we forget that this guillotine is hanging over our head at every second.