Where is it hitting you? If it is down about where a watch band would be or towards the base of your thumb, it's 99% chance you just need to raise your brace hight. However many long bow shooters expect to get hit there and just wear the armguard. I shoot a longbow and prefer not to be hit each shot mainly due to the noise it makes so I raise a my brace a bit till it goes away.
Another thing is how open or closed up your standce is. If you are getting hit in the middle of the forearm or near the elbow you may be turning to "edgeways" to the target and turning slightly more square will open up your stance and provide more clearance. Don't overdo it though. You should be comfortable.
Also, try pointing your bow arm elbow to the side. This will rotate your whole arm from the shoulder to the wrist and the natural bend in your arm gives quite a bit of extra clearance. I think it also feels stronger than the elbow pointing down but I've been doing it that way for so long anything else feels unnatural.
At the grip end of things, the centerline of your hand down at the heel should be a little left of center of the bow. When I shoot, the big bone in the heel of my hand that goes to my thumb is pretty much centered behind the handle. If you put the centerline of your hand down the center of the bow or even to the right, it moves your whole forearm into the path of the string.