I’m not sure what you mean by wondering about “the outcome” for someone who physically cannot cross himself in the normal fashion. All I can say is that there is no requirement that people cross themselves. It is a pious devotion encouraged by the Church, but there is no expectation that it must be performed by all, regardless of physical capability.
That said, perhaps it is simply the large, half-body sign of the cross that you are incapable to performing. If that is the case, you could modify your sign of the cross by using your right thumb to trace a sign over the left side of your chest (as is part of the gesture for when the Gospel is read at Mass). Or, you could place your first finger over your thumb to create a cross and kiss this cross (as is done in many Latin cultures).
