HD footage - Rubbish quality in export

Hi guys, am fairly new to Final Cut Pro 7 and having technical difficulties with something, I wondered if you might be able to help with?

Working on a 3 min vid. Have filmed the footage and edited it all on FCP7 in HD, yet when it comes to exporting, compressing (and uploading onto Youtube) somehow I am losing the quality of the vid and as soon as I save the project it is no longer in HD.

I realised earlier (only after i had edited all of the footage) that I hadn't selected the correct settings for the sequence I had been working on. I rectified this and altered the sequence settings to HD, yet this hasn't sorted the problem and still when I export the project and go to compress it the quality is greatly reduced.

Have you any idea what I may be doing wrong or why this is the case? Is there anything I can do about it? Or will I have to start from scratch?

Any help any of you may be able to offer me would be greatly appreciated!

Changing Sequence settings after the fact doesn't always work. Create a new Sequence with the proper settings. Then in the original, select all, copy, go to the new one, paste.

But more importantly, what is the format of the original media?
What are your new Sequence settings?
Who exactly are you exporting, what settings?