Maybe because the goal you aim is too high. Before going to work, you choose to finish the project so if you don't finish it but just 10%, obviously you will be disappointed.
Choose smaller step for your session. Explode the "Finish the project" task into more atomic tasks. Planning a project task by task is mostly made for organization, but also for self-motivation. Every step you will make will give you a feeling of completeness and will give you motivation to do more, this is how our brain works. In that way, you won't have feeling of struggling anymore.
Good luck, hope it helps !