You could try editing it in a text editor like Notepad to produce two separate smaller files, with half of the trackpoints in each. Each trackpoint will look something like this:
<trkpt lat="51.187781" lon="-0.6272">
<ele>83</ele>
<time>2019-09-19T00:04:45Z</time>
</trkpt>
This might make the files small enough to upload, then you can use the Combine Routes feature to join them together afterwards.
Sounds like a corrupted file, but I would try a tool like GPS Visualizer and if it does open it, reduce the points etc.
My Garmin Zumo XT has a recorded GPX file that is huge, 99mb. I want to view a portion of it, but unsurprisingly Plotaroute doesn't want to upload it.
Any suggestions as to a method to open it?