As far as I'm aware this is what has happened. Please correct me if I'm wrong.
The withdrawal agreement needs to contain a backstop, a guarantee by both the EU and the UK on how they will uphold the GFA which states no border in NI in the event that either we don't come to an agreement or the agreement does not prevent a hard border.
The backstop we agreed to was that NI would essentially stay in the single market, with a border within the Irish Sea. So if we diverge from EU standards sub standard products cannot enter the EU. This is only to be activated if we come to no deal or a deal that requires border checks.
The DUP objected to this as it created a border between NI and the rest of the UK. So we decided that having agreed to it, we would change it so that the backstop position of single market access would apply to the whole of the UK.
The EU then said that that wasn't what was meant, as it would then give the UK access to the single market without any of the responsibilities, and without free movement, which would be unfair to all the member states who balance the benefits with the responsibilities.They were willing to extend this to NI only, purely to uphold the GFA and in light of the history of NI.
The arch brexiteers didn't want this either, as it would mean that there would be less chance of the hard brexit they want, after all if we can retain all the benefits of the single market without the responsibilities, what incentive is there for us to strike a deal. Also it would make it impossible for us to deregulate any of our industries as we would still need to align with product and safety standards set by the EU. The hard brexiteers would rather break the GFA than stay in any way aligned to the EU, as they want deregulation and trade deals with countries such as the US, who have lower food standards for example.
So TM is trying to find a way to not crash out of the EU by agreeing a backstop, but because of her lack of majority in the house of commons is reliant on both the DUP and the arch brexiteers, neither of whom want a backstop that keeps any part of the UK aligned with the EU. Which is the point of the backstop.