Can I yell at my friend who has been belittling me?
What did your friend say to you?
He also insulted my girlfriend by calling her a whore while she was standing right there.
Your friend is really a rotten person for insulting you and your girlfriend. They are truly bullies, and it's great you aren't going to let them kick you down anymore. You need to keep standing up or else they'll only bully you two further.
But what if they go after me again and I won't be able to stand?
It is OK to stand up for yourself and not let your friends treat you with disrespect. Cutting your friend out of your life might be the only way to make sure you don't see them again and this doesn't happen.