How am I supposed to be comprehensive and help my girlfriend through her stress and ocd when she get mad at me, yell and smack me? I know she can't control it all and I'm pretty sure she loves me but still so hard to be there for her in those moments.

You are not obligated to "be there" for someone when it poses a risk to your emotional or physical well being, even for people you love. It's not ok for her to berate you or hit you. Sometimes the best thing you can do is excuse yourself from the situation.

Yeah you're seriously not obliged to put up with that behaviour just because mental illness is fueling it. There still needs to be boundaries and if she can't at least find ways to respect your wishes and redirect herself when she feels like going off at you, she's probably not in a healthy enough place mentally to be in a relationship. Definitely can help to tell her while she's not upset about how that isn't acceptable and that you won't be around her when she treats you that way and that it doesn't mean you don't love her it just means you're making safe boundaries. Her obsessive fears that you don't care about her don't take precedence over your safety. I know you don't want to do anything that can make her feel bad, but her feeling bad from it would be temporary, and if she takes it as OCD evidence for her fear then that isn't your responsibility. Here's a more extreme example: Two people go out on a date and go back to the guy's place afterwards. He wants to have sex and she doesn't. Maybe he's quite a sensitive person with low self esteem. She doesn't want to upset him. But to put herself and her own personal boundaries first, she needs to tell him no, even if it makes her feel guilty. Even in a relationship, you're not responsible for the other person's feelings, and certainly not to the extent of sacrificing your wellbeing to avoid triggering negative feelings. When she says no, it might make him very upset and might even make him angry and lash out. A guy who is emotionally healthy would accept the no without taking it personally and would take responsibility for looking after any of his own upset feelings the rejection might cause. That doesn't make the feelings her fault. Similarly, your girlfriend's feelings from you setting basic boundaries aren't your fault. Your safety and emotional well-being has to come first. You can't and shouldn't spend your life trying to make sure she never feels bad. You can love and support her in healthy ways without enduring abuse and sacrificing yourself. Allowing her to treat you that way won't do her any favours either. It'll not only damage your relationship and your trust in her, it'll normalise what she's doing in her own mind. It's not normal or okay. You have to teach people how to treat you. ESPECIALLY the people you love.