How do I calm myself before confronting my boyfriend?

I have been crying for the past hour about what's happened. It's probably nothing, but then again, I don't think it is.
I have been with my boyfriend for 9 months now. I've loved him for nearly five years. Overall, we are very happy together, and I love him to death.
However, throughout our relationship, there has been this girl who is his friends girlfriend. We never got along. She would flirt with my boyfriend in person in front of me and on messages, while being hostile towards me. She works at mcdonalds and threatened to spit in my drink next time i go there. My boyfriend and I went camping a few months ago, and she was sending him kisses, and I stupidly removed her from his phone. And I did that again a few months later. It was a stupid thing for me to do, but I finally communicated with him, and he said he'd be friends with her on Facebook but wouldn't add her again on snapchat. Since then, we have never been better.
Now, onto his friend. He's not a very nice person. I've been warned by multiple of my boyfriends friends and also friends of my own. He gets upset randomly and then tells us all to f off, etc. So I'm not very fond of him. Bit if he wants to be friends with him, then that's up to him.
So an hour ago I snapped my boyfriend, realising he's at his friends girlfriends house. And I broke down and had a full blow panic attack. The thought of him being with her literally sickens me. My dad calmed me down and explained to me that I needed to talk to him and find out if he's added her again. And if so, question our relationship. And I will, but the wait is horrible. I feel physically sick. I really want to know now, but he's asleep. I'm going over to his early tomorrow to clean his room and bring him his favourite biscuits. But I don't think I can face him, even if I have to. I know I have made mistakes with the situation, but too late to change them now. Any advice would he greatly appreciated.
How do I calm myself before confronting my boyfriend?
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