[chachi44089]

As for "beefing up" the area..Building up the weld is not the answer. That will just make the weld thicker, thus stronger than the base metal. The result would be another break right next to the weld. The weld need only be twice as wide as the thickness of the base metal, and 100% penetration. Have the two pieces beveled before they are welded.
To me the break looks more like a tear, starting at the web of the frame and headed down. Looks like excessive downward force on the bumper, or a lot of jarring pulling the mount down.
My advise would be to have a pro bevel and weld the break, then have the end of the frame capped and boxed with a plate of equal or thicker metal. That would greatly improve the strength That would distribute the stress more equally on the frame..not just all of the stress at the buttom. Also check the tongue weight of your trailer. If it is excessive "when loaded" you might need to move the axle on the trailer forward a bit. If it has light tongue weight, the movement "up and down" of the tongue when the trailer is loaded and moving may be too much. How much weight are you pulling? What is the tongue weight when loaded?
Hope this info helps!