I've spoken to my DH this morning about this. He said that at 18 he was much more like a 15 year old because of his ADHD (unmedicated then, hence why he could join the forces) and because his parents had never encouraged him to have friendships or relationships outside of the home. They dumped him with various babysitters he'd never met before so they could go out on day trips or to parties and they started leaving him and his sibling alone when they were 15 and 11 to go away to Europe for the weekend. He was relentlessly bullied by his older sibling, which they knew, but still left him in their care. They worked schedules which meant they were never there to see him to school from the age of about eight and then they weren't there when he got home from the age of about 11. He'd be left instructions for getting dinner sorted for the arrival of everyone else. His sibling was off doing whatever they wanted. He struggled horribly with it all due to his ADHD and his DF constantly telling him what a disappointment he was.
Anyway, he says he guesses they got tired of waiting for him to grow up and move away so that they could wash their hands of the parenting side of their lives, so when he got to 18 and got a job he loved in a restaurant, his parents didn't want him to keep it and pushed hard for him to join the forces. He didn't want to, but he had it drilled into him that he'd amount to nothing etc. He joined up, an 18 year old who had a reclusive childhood and few social skills. He was again, relentlessly bullied and felt suicidal, plus was seriously struggling with understanding instructions and regulations, which he told his parents about, but they were far more interested in his sibling who had recently emigrated. He went home on leave twice, then they emigrated halfway around the world to be near his sibling. For years he had nowhere to go when he had leave. He just stayed in his room while everyone else went home. His parents felt they did their duty by Skyping once a fortnight. I guess once he met me, they felt even more like he wasn't their responsibility anymore.
We probably get a FaceTime from them once every six weeks. Although he finds it hard to process and name his emotions, I think he feels he was really abandoned by his parents, and that the process started before they upped and left. And although the military way of life eventually suited him due to the routine, it broke his self confidence and his drive to make friends. I place that squarely on their shoulders. They should have known their boy. They should have known he would struggle and they should have been patient and helpful- waiting for him to find his feet in the adult world and guiding him toward independence while he did the job he loved. Instead they crushed his emerging happiness at finding a social group to fit into and they badgered him into an unsuitable life and left him all on his own. Cruel.
I think your parents were cruel too, OP. You don't do that to your kids. Abandonment is the height of selfishness and it leaves such deep scars. I'm really sorry for 15 year old you- you deserved much better care.