> Struggling to finish project, how to tell coworker?

At this point you need to be open and honest about the status of the project.  Honesty and transparency are usually your best ally when it comes to a project's status.  ( Just tell the co-worker **as soon as possible** )

You have made a mistake in not disclosing the status of the project, but one that I think you can recover from by being honest about it.  Show your co-worker where the project is at and ask for the best way to proceed.

In the future, *as soon as you are struggling with a project*, let whoever is in charge of the project know so **they can plan accordingly** and have whoever is in charge prioritize what they want to see finished first.

Free advice about development:  Be sure you break a task or feature down such that you can complete *each small part in a 2 or 3 week time frame*.  This will allow you to see much earlier on if your project is in trouble.

Best of luck to you.