Build a wall and put a door in it.

Build a wall where you have the curtains. I'd go a step further when you have the money and renovate the space so it flows better by taking out that wall between the sink area and the shower/toilet too.

The sink also looks built in so when you replace the countertop you'll have to get a new sink too.

Removing the decoupage is your best bet, but be prepared for the countertop and sink to have some damage that was covered up. Peer under the sink to see if the sink and countertop is one piece of cultured marble. You can experiment with solvents to see what the decoupagers used to seal the paper underneath. If the previous owners did it, you can ask them.

IMO a new counter and a new sink is in your future but honestly I would not spend much money until you have a plan about the whole bathroom space . I agree a wall with a door where you have the drapes is a good stop gap measure until you can redo the whole thing.

What did you decide on from this old post? I have the exact same dilema. I was going to do curtain but wasn't sure if make it too closed in. Mine smaller 3 1/2 ft from sink to opening.