人就是這樣， 越得不到的東西就越想得到
( Human is like this, when you couldn't get something, you want it badly )

Once you get it, you will feel like " Awww I got it like this, Well, I'm going for another one.

That explains why you feel NOTHING when your exgf begging you to stay.

Crush vs Love, do you want to risk ? But I can tell you 99% you WILL regret for choosing your colleague.
And the reason why you fall for your colleague because you meet her EVERYDAY, but you gf only on the weekends.

Gf is a awesome girl and will be a great wife but you dont marry someone because of that, you marry or be with someone because you LOVE her.

I guess the only way for both of you to settle this is,

Get away from all these sh*ts, go somewhere FAR away, a vacation.

A vacation = at least a week, not those 3 days 2 night trip.
You will get your answer in this period.

If you think it's not neccesary to do that or you decided to end this relationship, then stop think about your gf's future.
Whether she's happy or not, it's none of your business, you are the one who choose to left her and end this 3 years relationship.
The more you concern about her, the harder for her to move on and has a new life.
Stop distrubing her.

Otherwise, do something if you really love her.
And please be prepare after both of you decide to get back together, you will need to work EXTRA hard to build back this relationship and the trust between both of you.
She will be 24/7 insecure.

Relationship is like this, it goes up and down, if you can overcome all these problems, she's the one for you, you're the one for her.