When DD2 was newborn she copped a few whacks from DD1 the only thing I could do was distraction...nothing else worked. It did pass and now DD1 is nice and loving to DD2 but we still have the occasional issue.
DH also make the effort now with DD2 so I can have some one on one time with DD1 which helps her behavour heaps.
Dont ever feel you have failed him you gave him a little brother and one day im sure they will be the best of friends