Make no mistake, Steve mourned. In his alone, in his depression, before Bucky was alive again, Steve mourned him. He mourned everyone. He mourned Peggy, he mourned the Howling Commandos, he mourned Howard Stark, he mourned New York in the 40s. He had no closure. He had no funeral. In fact, no one really recognized his mourning, either. They didn't understand, because how could they? Steve didn't expect them to, as he gave up trying to drink and instead, wallowed in his sadness. He didn't have friends anymore, he knew. Just acquaintances and Captain America fans. So he mourned his relationships turned to dust and his life, gone haywire, and sat in his devastating aloneness to consider his meaninglessness and his survivor's guilt, and let it eat away at his very heart until the guilt, well…
The guilt destroyed him.