I am in a 5 year relationship with my girlfriend...

I am in a 5 year relationship with my girlfriend. I started a new job nearly 2 months ago and have become very close with a female colleague who started at the same time as me.

We have the same humour, same interests, and we can intellectually stimulate each other which I don't get with my current partner. I have developed feelings for this colleague and I look at my girlfriend differently now, it feels different when I am with her.

What do?

Find if this girls acts different outside of work, around friends, etc. then if it feels like you deal with her long term you have to let your current girlfriend go easily and maintain composure. Give yourself a month in between the two so you have some self reflecting time.

You're retarded. Completely retarded. Its like one of those comics where the x speaks and the other guy says oh its retarded.

fuck them both nigger

Weigh the pros and cons of the situation. Do you value the stability and life you two have built as COMPANIONS? Or are you craving this new love feeling of something new and unexplored? You must have felt this way about your girlfriend in the beginning, no?

Don't cheat. Don't do that to anyone. It's okay to have these thoughts, but if you're going to mentally betray her with falling in love with someone else, you better be dedicated to it or seek help from a professional that can give advice to you two as a couple.

If it persists, and you've made arrangements on your exit plan and weighed your pros and cons, make sure you break up with your girlfriend. Don't lie about anything. She will find out and she deserves to know.

Just. Don't Cheat. It destroys people.

Remember that episode of the Simpsons when homer met mindy? That's kind of like your situation.

We have been out at staff do's and the like. It's the same, we get on like a house on fire. I know it's too soon to know anything and 'the grass is always greener...' but I am worried about where this is headed.

I think there's a lot of sense in that, that the novelty of this attractive person who interests me and (seemingly) vice versa.
Wouldn't cheat on her, ever. That's out of the question.

I don't believe on "mental betray" though, I didn't choose to feel this.

Kek that's very true

how about you get over your week long crush and think about why you soent 5 years with your current gf

Good luck.
If you break up with your gf to date this new girl, my experience and what I've gone through with friends shows is very unlikely to work out. Relationships with people at work are fun but don't really last once the novelty wears off. They add a personal and exciting element to work, kind of like a special secret. But that kind of escape doesn't make the best foundation for a relationship. It needs to evolve into something meaningful before the excitement wears away (which can be hard top gauge an interest in from tree outset), or it risks falling apart.