Great advice.  If anyone received a superabundance of graces, it was Judas, who spent nearly 3 years in close proximity with Our Lord Himself ... and yet he too, in the end, exercised his free will to reject them all.

In terms of "wielding" God (others have likened using God as a vending machine), I've always remembered the story told by an early Church Father.  So, there was a man fishing on a lake, and at one point he catches something, which must have been very substantial due to the resistance on the line.  So he pulls with all his might for a long time, and finally sees his catch drawing closer, eventually realizing that his line was caught on a large, immovable boulder, and while he thought he was reeling it in closer to the boat, the entire time he was actually pulling the boat toward the boulder.  That is what prayer is like.  We don't "reel in" God, but merely draw ourselves closer to Him.  Even if OP were to pray, fast, do penance hard enough to "win" the conversion of this soul, it was God Who inspired him and gave him the graces to do it in the first place, i.e. it's always God and only God who converts.  God desires the conversion of this individual more than OP does, and more than the individual does, infinitely more.  This doesn't mean to be complacement, so you should do everything you can, but always realizing that it's God insipiring you and giving you the grace to do it.  God's economy of grace can also be likened to young children who have no way to earn money, can't get jobs, etc.  But the parents give the children an allowance.  These children then save their money for a long time and buy the parents a gift.  Ultimately, the parents were the source of the money, and in a way financed this gift, but what is moving about the gift is the generosity of the children in saving the money for their parents rather than spending it on themselves.  Any graces that may funnel through us to others originate with God in the first place.

If you are sincere in your prayers God will surely answer them, but maybe not in the way you want them to be answered. Remember, He will always do things in the most perfect way possible.