Anyone can do some of those things now and again. Healthy people -- when realizing they have trespassed on another -- APOLOGIZE and are remorseful and make amends and strive to not do it again.
If it is a cluster of things, chronically happening, without remorse -- red flags should go up. And some things like hitting? That doesn't get any second chances. At least not with me.
The best scenario would be for their relationship with each other to naturally fall apart without me, as I guess I was hoping for. I want him to make the decision not to be with A on his own, either from respect for me or self preservation for himself.
I see you want that. But you cannot control B or choose his behavior for him. You can only choose your own. If you stay with him after this incident with A hitting you? What does that teach B about how you want to be treated?
That you are ok with him dating a guy who hits you? That is not cool. It is not ok. It is not kind, loving behavior toward you for B to be "ok" with you getting hit by his other partner A. It is not kind, loving behavior for A to hit his partner B's other sweetie. It is not kind for your partner A to hit you. Not matter which way you slice it -- it is not good.
I feel very much alone I don't want to lose B, I've never been in love with anyone in the same way so really want to make it work with him. The problem is, B still feels the same way about A.
I know you love him. But "loving someone" doesn't mean automatic "staying with someone." Especially in a potentially damaging situation -- sometimes it is safer to love them from a safe distance.
Healthy loving is not hurtful.
Not mentioning this to B wouldn't make me physically unsafe, but I do plan on talking to him at the end of the week even if something comes up or he says he is too stressed to hear it.
I'm glad you plan to talk to B soon. And I hope you tell him exactly what you have expressed here -- you are not comfortable with him and A together, especially after being hit by A.
I hope he chooses you, and gets away from A (who by your account sounds kind of user-y).
I hope if he doesn't choose you, that YOU will choose you.
And that you walk away from potentially harmful relationship(s).
A has clearly harmed you physically. I don't see how staying with B (knowing that he is seeing your assailant A) is anything but mentally and emotionally harmful/unsafe to you. It already bothers you that he did not try to defend you in the heat of the moment or called A into account or anything. It is devaluing you.
If he doesn't want to hear it, or gets avoidy -- that too devalues you. It is not good for you to stay with a partner that is emotionally unsafe. You doing something like that is YOU devaluing you. YOU are responsible for caring for you mental health, emotional health, physical health, and spiritual health.
You don't need this crap at 19 -- you don't need it EVER. You deserve to be treated well. You do NOT deserve to be treated badly. You have worth, dignity, and value.
You deserve better treatment than this.
That all may be Hard to Hear, because you do love B. But again... sometimes it's better to decide to love from a safe distance. Loving someone does not mean automatic staying with them.
Physical bruises are not the only things that happen -- emotional and mental bruising can happen to.
I am sorry you are hurting and going through all this. I hope you decide to take care of YOU first and foremost. There will be more loves in your life -- if you are Open to allow it to come in time.