This raises more questions than answers, but just in case it helps, here are some issues that occur to me.
Of course 18-year-old young men can be victims of rape or assault, but if they are legally considered adults (which depends on the age of legal consent for men and women in a given time and place), it would not be considered statutory rape, child abuse, etc. That would be a factual question about where Jung was living and when this supposedly happened.
The other question is what the Attentat might have actually entailed, in what circumstance he might have encountered the man. In the current modern climate, trying to root out sexual harassment in the workplace, just unwanted touching, which is indeed unethical and wrong, is culpable enough to sometimes be lumped together in a broad category called abuse, and to an inexperienced young man might indeed have been an emotional shock. But legally, in many jurisdictions, there has traditionally been a difference between mere unwanted contact and the more serious offense of abuse involving force or violent restraint; and yet another difference if the abuse involves penetration, i.e., rape. (Interesting that several of these aspects are currently in the background of the news surrounding accusations against Catholic clergy. It does make a difference whether the victims were younger children, who would be more subject to psychological threats, and whether there might have ever been an element of consent on the part of some older teenagers or young adults. But precisely those details may vary from case to case in a way that generalizations in the news simply can't cover or even know.)
But anyway, since the word 'Attentat' actually seems uncommon for any of those possibilities, it's hard for me to guess what exactly he might have meant by it. LEO translates it only with phrases including the word 'assassination,' i.e., murder of a famous person, which matches the modern Duden definition. It seems unlikely that it literally felt to him like an attempted murder, and while he always had a healthy ego, it also seems unlikely that he was so megalomaniacal as to imagine that he was famous.
Could he have had in mind some older sense of the word, like 'versuchtes Verbrechen' in Duden? How much older was that?
Or could he have simply been alluding to the idiom, which Duden defines only as wanting something particular from someone? That might actually be more plausible, using a consciously overstated, humorous word for something that might have seemed huge at the time, but considerably less earth-shattering looking back from adulthood, perhaps decades later.
And what about 'unterliegen'? That also seems unusually, er, vivid in this particular context, though I can't think of any way to translate it literally enough to get that aspect across. Maybe something like 'I was subject to / subjected to an attack'? Could the image of lying under the man also fit with a witty retelling, making light of the incident, or appearing to make light of it in retrospect, in order to impress Freud with his urbanity? Or should we simply take it as a clue that the original event was indeed more serious, and therefore more traumatic, than just being propositioned or felt up?
Sorry, I realize none of that actually answers the question of how to translate it more vividly. If the exact wording matters so much, maybe it deserves a discussion in a footnote.