Is it normal to be bad at arguing and then think of alot of things to say after the argue has ended? Everytime my sister or my gf wants to beef I can't bring myself to say hurtul things so instead I let them say all the stuff they want and what I just say back to them is I'm sorry I'm sort or mmhm...

Try and stand up for yourself, bottling things up isn't good, it'll turn into deep resentment.
If you are no good at confrontations that maybe try waiting until they have calmed down and try and explain how you feel.
Tell them you can't deal with angry nasty arguments and that if they actually want you too understand how they feel, they'll have to do it calmly.
If they truly deserve some shit flung at them, you can apologize later. Don't do it before.