I'll second the photos suggestion. I've taken three sets so far, at 17st 9, 16st, and 14st 9. It's amazing looking at them and seeing all these changes that I just don't see when I look in the mirror. It's also amazing looking at my back fat in the most recent set - I hadn't realised how much I had until I saw how much has gone!
High weight clothes are another good one. My stomach looks the same to me now as it did fifty pounds ago, but some of my vests are
gaping around my stomach now. So either it's smaller, or someone's being altering my clothes while I'm not looking.
You still see a fat person in the mirror, because, despite an incredible loss (well done!) you're still technically a fat person. It sucks, a lot, and sometimes it makes it feel like this is going to take forever. But it helps to remember that fat isn't just one size, and even though you've still a fair way to go, you really are a lot less fat than you once were. And that's worth celebrating, I reckon.