Help fixing jagged text

Hey folks, hopefully one of you can save my sanity with this one, as I cannot find a solution elsewhere. The title is pretty self-explanatory; here's the details. 

This first pic is a screengrab of text in my program monitor. Looks great, right? Nice and smooth. 

Next up, the same, but in the export preview window (the rendered video looks the same)

It's pixelated to hell and looks awful. 

Can someone please tell me why? I assume that it has something to do with my export settings. Here's the summary of those; as you can see, the export matches the source - 

Please, someone put me out of my misery. 

Thanks in advance!