Most traditional bow poundage is based on a specific draw length, 28 inches is most common. Given your size, it's likely your draw lenght is less than 28 inches, which means you would be pulling less than 50lbs. It is my opinion that the bow is still too heavy for you. A comfortable, easy to pull draw weight is critical. Hope this helps.
I would look into buying a cheaper 30 - 35 Lb (possibly even less). bow to get your muscles used to pulling weight. Shoot this bow once your muscles can easily handle a 30 - 35 Lb (or less). Perhaps after you've gone through a nice long warm-up with your lesser poundage bow, you can pull your 50 pounder a few times.
If it is really too much to hold in an anchored position, I'd back off until you're stronger.
I'd certainly not give up on the bow that was given to you. I'd just work up to it. It could take a while.
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