I dont know how to feel?

Right.
I was dating a guy, it didnt work out, we ended it, two weeks later we kinda started seeing each other again, then we ended it again. We are remaining friends, In the two weeks we werent together he slept with my best mate, i found this out after we kinda ended it the second time.
This was the first guy iv ever dated or had sober sex with. My best mate knew this, she knew how paranoid i was etcetc, i heard all this from him and not her, i feel betrayed by her, and hurt, A that she didnt tell me, and B that shed sleep with someone she had no feelings for, knowing i had feelings for him, and knowing how messed up i felt about it, do i have the right to feel hurt?
Would you forgive her? Shes had goes at me for less, once her ex took my to hospital to get stiches and she sent me texts saying do i mean that little to you, etc. I just dont know what to feel