Hi, my ex-girlfriend of 4 years and I have recently broke up. It’s been about a month and a half and I'm still a bit shook up. After the break I found out that throughout the relationship, she hooked up with other partners. That hurt a lot, but I'll get over that, just like I'll get over her as well. My question is how? How does someone justify that to themselves, while insisting on how much they "love you" and "need you"? Before I found out that she was fooling around on my behind my back, I saw us being friends in the future, but now (and this may be petty) I just despise her.

Now I'm worried that ill have trouble trusting other people in the future. How does someone overcome that? How does someone learn to love and trust again, after a betrayal like that?

First of all, not everyone is like this mate, I've heard a lot of stories as yours, I know there will be some times in life that you will have hard time to trust someone. But the girl you have been with is fake, she doesn't know the meaning of love, there are people like that, but don't worry, you will eventually find people who you can trust.
There's a big world out there mate, there will come times when you will hopefully give up, but you just have to keep your head up and handle it.

All I am saying is that you shouldn't give up.
Good luck mate :)

We get our strength and confidence back. It takes time but we all go through this. We just have to regroup, and build a life that you enjoy, with family, friends, and activities that you enjoy, and rebuild the faith you have in yourself to deal with what ever life throws at you.

Just takes time and a bit of work on your part.