How do I achieve reliance upon Allah to marry a specific girl, given the obstacles preventing me, along with my certainty that Allah is the Lord of means and that He will suffice me?

Reliance on Allah is a great act of worship stemming from trust in Him, which arises from knowing Allah through His names, attributes, and actions. Reliance, then, is the heart's dependence on the One relied upon. If a servant becomes certain that there is no doer of deeds other than Him, and He possesses perfect knowledge, power, and mercy, then his heart will rely on Him alone. If the servant does not find himself in this state, its cause is weak certainty or a weak heart. Allah knows what is good and righteous for the servant and ordains matters with wisdom and mercy. He may turn away from him something he desires out of mercy for him, as Allah Almighty says: "And perhaps you dislike a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah knows, while you know not." (Al-Baqarah: 216). So, entrust your affairs to Allah and supplicate often, for it benefits the servant in all circumstances. There is no Muslim who supplicates except that Allah grants him one of three things: His supplication is hastened for him, or it is stored for him in the Hereafter, or evil similar to it is averted from him.