Hi and welcome :) I competley agree with Lol and Zaf... We all have moments of curiosity and we all do things that we regret later on. I think Lol is right in saying that a chemical imbalance may be affecting your perspective on this situation - and that you have overthought it so much that it has escalted into becomming something that is actually much bigger than it is. I have been with my boyfriend for over 8 years, he's 27, I'm 26 - if one day he confessed to me that he had done what you had... I certainly wouldn't think any less of him! Look at this in a rational light... 1) You were ignoring the ones that looked dodgy and were watching normal porn 2) You were curious, young and in the 'heat of the moment' that curiosity got the better of you 3) You previewed it - If it was actually what the title said, then I doubt very much going from your posts that you would have actually watched it - as it was, it was fine! You do not get turned on by this sort of thing, you weren't set on looking for that sort of material and you have beaten youself up for so long over one simple, single mistake that its obvious to anyone reading that you are quite rightly replused by this sort of material.
I definetly think you should go to your GP and try and get some treatment for depression sorted out - its up to you whether you tell your girlfriend or not, only you can really judge that, we'll all be here for support and a friendly ear if you need it :) Nay x