Your boyfriend is abusing you - emotionally and psychologically, and physically. You need to break up with him AND his friend group.
Based on your OP, it seems like the friend group knows he's keeping you from joining them. They are either happy that he's doing it, or they don't care that he's hurting you. Either way, they are not your friends.
He's also cheating on you wit his female best friend. The way he's acting, she is his priority and the person he wants to be with. He's treating you like the side piece. Actually he's treating you like a one-night stand that won't leave him alone. His friend groups knows, they don't care, and are actually enabling his cheating on you.
Life lesson: No one is going to fight for you, if you are not willing to fight for YOURSELF. Maybe things would have been different had you broken up with him when he first started abusing you. But you just keep taking it. You get mad, but YOU STAY.
The message is clear, you have NO self-esteem and NO sense of self-worth, you are just going to let him use you as a punching bag - emotionally, psychologically, and physically. You have never held him accountable for his abuse, and he has never faced any consequences for abusing you. Why would anyone in the friend group go against him and defend you? Why would anyone in your friend group risk getting kicked out for trying to help you when you have demonstrated time and time again, that you won't leave him?
I know am being harsh. I'm not blaming you or excusing the behavior of the friend group - they are despicable. Everything I've said is the truth. You have to fight for yourself. You are too comfortable with the victim role. It's clear you are used to being a victim.
You told him you would be going home and would not be at the cake cutting because you were hoping that he would finally like/love you enough to say, "don't go, stay and celebrate with us". That's a ridiculous expectation given that he literally dragged you out of the last get together. You betrayed yourself when you let him know you were going home. You victimized yourself ALL over again by handing him a chance to reject and demean you again.
This comment is too long already, so let me wrap this up.
• He doesn't like you and he certainly doesn't love you. He's also cheating on you with his female "best friend".
• He only keeps you around to abuse you because abusing you makes him feel powerful.
• Break up with him, TODAY.
• You don't need to have some long, drawn out conversation with him about you, him, or the relationship.
• There is no such thing as "closure". That feeling of things being settled, and you being at peace with what happened, that's something ONLY YOU can give yourself. No one else can give it to you.
• Don't try to be his friend. Cut all ties and move on with your life.
• You don't have any friends in that group of people. Stop begging for attention, and acceptance from people that don't like you. They won't even treat you with common human decency.
• Get new friends, and block everyone from your boyfriend's friend group. They enabled your boyfriend's abuse.
• In general, start demanding to be treated with RESPECT. If someone doesn't treat you with respect, WALK AWAY.
• You need the help of a trained professional to help you address your lack of self-esteem and lack of self-worth. Start individual therapy as soon as possible.
Hope this helps.