Premier? Premier Elements? Which version? What computer and computer specs? What camera? And, "upload" from what to what? You'll get lots of help here from other users if you can take the time to write some detail about what and how your are doing it. Nobody can guess.
The answers are in the details. So let us explore them.
Please confirm if your problem resides in editing the .mts files already imported into the existing project or if the problem is that you cannot import additional mts files into the project. How large is the project when you have the Timeline editing problem (duration and complexity)? And, what specific editing task can you not do? Or, are you having problems opening a saved closed project that you have been working with.
Answers to the above will direct the troubleshooting strategy. The following routine background information is the frame work for the troubleshooting details.
1. What version of Premiere Elements? What computer operating system?
2. What are the properties of these .mts files? If you do not know, often telling us the brand/model/camera that recorded the video will give us the troubleshooting information that we seek. What is the project preset set to match the properties of the source media (See Edit Menu/Project Settings/General.)
3. What effects, titles, and transitions have you added to the Timeline files? And, how complex is the Timeline structure?
Let us start here, and then decide what next. brandonb, if you have any questions or need clarification on the above, please do not hesitate to ask.