Iran says a deal with the US is achievable if the aim is to prevent nuclear arms
This comment shows why it's so hard to make a deal. It's so tough for Iran to trust that the US isn't trying to disarm them as a first step towards regime change. But the US can't trust Iran because it's building a nuclear weapon.
Ultimately what happens probably comes down to one person -- Trump -- and his willingness to believe Iran or his willingness to go to war.