I think the translation is good, but it does not translate the en. Maybe:
I am still beside myself because of it/that,
That still has me quite beside myself.
that en should be somehow reflected in the translation.
Être hors de soi means, according to CNRTL: Être très en colère, and the following example is given:
Nous l'avons vu arriver rouge, hors de lui et dans un état d'agitation extrême (Ponson du Terr., Rocambole, t. 1, 1859, p. 131).
So, this expression pertains only to anger, and could not be used, to convey, e.g. that the person is overcome with joy, but it seems that beside oneself has the same meaning in English.