Feeling utterly garbage right now.

Well where do I start.

I'm the biggest idiot on this planet.

I'm in a relationship of 8 years this year with my girlfriend. We don't live together yet and see each other mainly only at weekends.

I've been using a car forum where I got talking to this other girl. We had a lot of similar interests. We chatted a lot in the main part of the forums and then in private messages. She gave me her number and I gave her mine. At this point which I may add, it was nothing more then just friendly chatting.

Soon after hundreds of texts were sent back and fourth we got talking about sex and other stuff. Pictures and videos were exchanged.
Don't need to go into detail there but she had my full attention basically.

She lives around 3 hours away down south. She told me shes heading up near me for a night out with work and would love to meet.
I at this point having really started to like her agreed.
Then she said it was not happening, but Friday just gone she said I'm coming up tomorrow if you want to see me.

Next day we spoke the day we met I initially declined thinking more with my head that it was a bad idea. At this point we'd already told each other we both really liked each other.

Anyway I bit the bullet and went to meet her. She was in a hotel room. I had major butterflies thinking oh god! As cheesy as that sounds.

We layed on the bed watching formula 1 and just chatting for around 2 hours. It was going great actually we really got on. Then we just looked at each other and I found myself kissing her. We kissed quite a few times but that was literally it.

After I left around 30 mins later I've hardly heard from her at all. She says nothings wrong and texts me randomly hours after I've last spoke to her.

My mind at this moment is in bits. She seems so carefree and I've found myself listening to her favourite song over and over staring at my phone.

I feel utterly terrible for what I've done to my girlfriend. I'm the biggest arse hole on this planet.
But I have no idea what I'm doing at the moment. I cant eat, I cant sleep and I find myself thinking about this other girl all the time.

I don't want to feel like this about her.

Has anyone been in a similar situation or got any advice?