In hindsight, Shiro didn't know what he had expected when they returned back to Earth. Maybe for Adam to welcome him back, somehow still in love him like Shiro was. Maybe to know that Adam had moved on and was happy which was Shiro wanted for him the most. Maybe that Adam would still be upset with him but they would be able to reconcile and get back together.
He definitely hadn't thought that the Galra would attack Earth before they got back and Adam would be killed in that attack before Voltron could come to the rescue.
When he hadn't seen Adam upon returning to Earth, Shiro had just assumed that Adam was stationed somewhere else in the Garrison. Adam had been discussing with him how he would prefer to retire to teaching as soon as he could, because he wasn't passionate about flying like Shiro was. They had still been mapping out their future together at that point.
It was actually a full day after they arrived and found out about the details of the Galra invasion that he was told of Adam's fate and that had been a minute of complete disbelief and denial. Of all the ways that they would end up, this wasn't the way Shiro had seen coming. And he could feel his heart shattering once more as he grieved the loss of the man he loved again.
He wast too numb to react much at first. He just vaguely nodded at Sam who had been the one to tell him about Adam and left to go check on the paladins and Alteans. Once that was done, he was met by Curtis, part of the crew who would work on the replacement for the Castle of Lions, who needed him to confirm some details about the blueprints. It was only at the end of the day, when he was at the room the Garrison had assigned him and had finished dressing down that he let himself think about Adam again (his smile, his laugh, his eyes, training together, dating, when he had proposed, how they had broken up). He'd had a lot of practice suppressing his memories of Adam those last couple of years so it hadn't been that difficult to bottle it up until now. But now the memories resurfaced with a flood of feelings that made him feel like he was drowning.
As Shiro let himself remember everything that he had once forced himself to forget, he finally broke down and cried. Cried at the death of one of the most wonderful people he had ever known. Cried at losing the love of his life again. Cried at everything that they could have been together that was taken away from them by his disease and the Galra empire. He didn't remember when exactly he cried himself to sleep that night, but he eventually fell asleep-finally letting go of his denial of the end of his love with Adam Wilde.