<human>: Can you summarize the given text in a few sentences?

About 2 months ago, I broke up with my girlfriend of ~2 years. Now, one of my closest friends is seeing here. The reason I broke up with her was that I was stressed out with having a relationship and felt that I wasn't giving her enough attention as she was giving me in the relationship. I did not want to string her along in a relationship that I just couldn't commit as much to.

Fast forward a month and a half later, my friend asks me if i have some time to talk. He says to me "Me and Ex have been hanging out. I just wanted to ask you if its okay because you are one of my best friends." I am an adamant believer in making others happy even if it makes you unhappy, so I told my friend it was okay. Furthermore, my friend is one of the nicest people I know, and I know that he will treat her very well and make her happy. 

All I care is that she is happy. However, when I see them together, I get this gut-wrenching feeling, and feel very depressed after. I think this is because its natural to feel upset about a close friend seeing an ex, but I am conflicted because I want her to be happy more than anything. Has anyone had experience with a similar situation? I'm having trouble dealing with it and would appreciate any advice at all.
<bot>: Best friend is seeing ex after I gave him permission. I want her to be happy but it kills me seeing them spend so much time together