The key problem in the Brexit deal remains what to do with the Irish border. In essence, the UK faces the same trilemma it has since the beginning. It wants three things it cannot have at the same time: exiting the EU single market and customs union, so it can create a free and independent trade policy; no hard border between Ireland and Northern Ireland, per the Good Friday Agreement that ended the Troubles; and territorial integrity, i.e. no hard border between Northern Ireland and the rest of the UK in the Irish Sea.
Essentially, the UK can choose two of these three options. Below, I have attempted to show how they relate and what the result is once two of the options have been chosen.
Credit for the piece is mine.