In a real pickle, help?!?

I feel really embarrassed typing this but I think when I was about 9 out of curiosity I started looking at porn. I didn't get addicted or anything, just occasionally looked at it when I was home alone or something. Well when I started high school I looked at a porn website like videos I guess hardcore porn. I always thought it was wrong I guess but I just thought, "nobody else knows and it's natural, isn't it?" and that is also another question I'm not sure about, is it natural, (like at this adolescent stage)? So from then on I just did the same thing, I watched porn if I was bored or something like that. I guess I became a little addicted but never really had the urge to watch it just whenever I was home alone, bored or felt curious. So anyway on the weekend just before I turned 14 (my birthday was on the Monday) my mum found porn on my brothers laptop history, although everyone in the family uses it and he has his own account on it. So on the Saturday afternoon a while before we were going to go out that night for dinner for my birthday, mum called me downstairs. I thought it would just be something small. I sat on the arm of the chair and she stood in front of me. She said something like this, "I looked at the laptop history and was very disappointed to see that there were porn websites there." As she said all this I was looking her basically in the eye the whole time and just froze, got goosebumps and my stomach dropped a mile. It was the one secret I had, the one little secret I had that I thought nobody would ever know, the thing that I thought was harmless had just been revealed. And then because I was scared sh*tless I looked her in the eye and said, "It wasn't me". Then I couldn't tell if she believed me or not and just said something like "I'm putting the laptop away". I walked off then and went to the toilet and just sat there feeling sick, it was painful. I couldn't even really enjoy my night out then, I felt scared to leave her alone in a group worried of her telling everyone and I was scared to be alone with her as I thought she might bring it up, and my grandma was there and I love my grandma so much, I couldn't bare her knowing. The next day while the both of us were eating lunch at the table, just the two of us I just kept talking not allowing her to touch on the subject, it must've been so obvious what I was trying to do, but I was scared. There was one other time she mentioned it but I just said it wasn't me but it was more just through conversation so I could easily change the subject and stuff. The other thing is that during this stage that I watched porn I was watching gay porn as well. I had been having a bit of a dispute with my sexuality. But now 4 months after when my mum found out, I've vowed to never look at porn again (I haven't and I know I won't), worked out that I'm straight and fallen in love with the most perfect girl. So basically I've got my life together but quite often I always think that I haven't confessed to my mum and that I lied to her face. I just can't bring myself to do it and if I do like if she saw that some of those sites were gay, she might think I'm gay but I'm not. She will also think I'm a dirty pig. You might say I should've been smarter and deleted the history which I always do but you must know that this was just one days worth, I was more just browsing around on heaps of sites. So now my life is awesome but this one thing is hanging over me but I've given you reasons why I can't tell her. And lately our relationship has been normal and if I told her wouldn't it just ruin it? And I've given you reasons why I can't really tell her. I've turned around my mind, I'm not perverted, I never call a girl hot or anything like those jerks at my school. I've never had a secret really, this was the one secret and I fell so very bad! Please help me! Thankyou.