Maybe PM Marguerite, I believe she's a native...
From my limited knowledge:
Estoy aquí para usted. - I am here for you, Sir/Madame/my Lord/my Lady - addressing someone of higher stance than you and you are at the same time acknowledging your inferior stance. Using usted is a very polite and formal way of addressing someone. If it's a close friend, then it's "Estoy aquí por tí."
Likewise: ¡Cortaré sus bolas! and ¡Cortaré sus cojones! is again, addressing Sir/Madame/my Lord/my Lady. Since it's supposed to be a swear or threat, I'd use tus bolas or tus cojones.
Soy aquí para tu. Spanish has two verbs for "to be" - ser and estar. Ser is used in relation to things that cannot be changed, whereas estar is used for the opposite.
Estoy aquí - I am here, as in right now in this place, but in the next minute I can be outside. Estoy sitiendo - I'm sitting, but I can stand up.
Soy Checa - yes, I'm Czech and that's not changing, ever. Soy una mujer - I'm a woman, again something that's not changing in a hurry (sure, modern medicine miracle and all that, but for grammatical purposes it's not changeable).
Soooo... facing a dilemma of "I am here for you," I think I'd rather use something I'd be more comfortable coming up with.
"¡Déjame ayudárte!" - Let me help you! possibly accompanied with Habla (conmigo) - Talk (to me) or more direct ¡Dime! - Tell me!
Other soothing line could be "No te preocupes." - Don't trouble yourself.