Disaster with Mpeg Streamclip!

So I am definitely not an expert so don't burn me at the stake if I am doing something completely wrong - but I am so stuck! I've been trying to do some editing for a college project and I really need some film clips in HD. The main one I am having issues with was downloaded as an mpeg4 file, but because it was so big it wouldn't fit on fcp and I had to find some way of cutting it down. In walks in Mpeg Streamclip!

I've used the programme once before to import footage from a dvd and it all went swimmingly! But after an all nighter trying to work with an extremely unresponsive and jerky streamclip, trying to cut out all irrelevant footage, it then froze up, crashed and completely deleted everything I'd done.

So before I started all again I thought I'd check to see if it would allow me to export it (just in case I got to the end to find I got error messages). Lo and behold, every single different format I tried to export it as I'd get the same 'Can't prepare movie' message! And I don't know what to do! I've done all the fix timecodes and stuff, but my brain is literally like marshmallow now and all other forums I've found only have explanations that I can't understand!

So if anyone has any clue how I can fix this - It would be most appreciated!!