Why are you doing this by hand, kokopuffs? If you have a mixer. I';ve done it often because i often cook at other people's houses who don;t have mixers, and also i didn;t have a mixer the first years here, and i made everything by hand. I have very strong hands, though.
I learned some tricks, which i can share.
I never (i mean never) plan in advance enough to keep the butter out to room temp. So i would always heat the bowl by running hot water on the underside of it (not getting any inside).
Then i'd begin by squashing the butter with a fork, or cutting it up, and then squashing it with a fork.
If you have a microwave with a watt setting, set it to 90 and get the butter soft that way, but i only got a micro about ten years ago.
when the butter is somewhat squashed against the bowl, hold it sideways so the hot water runs over the bottom only, or put it a moment quickly into a pot of hot water, like a bain marie, but only for a second. It will melt a little of it but you mix them together and it becomes soft butter.
Then (THIS IS VERY IMPORTANT) don't beat by hand on a kitchen counter, or your arms will ache more. Put the bowl on the kitchen table, that way your arms are not raised and you don;t have to hold them up as you beat, making it doubly hard work.
Sometimes i whip cream or egg whjites by hand just not to bother with the mixer, or if the bowl of the mixer is dirty or being used. But always on the kitchen table)