Sekai no Chuushin de, Ai wo Sakebu, Wasurerarenai Recommendations
Same problem...A story about loosing a dear friend and old love.
Wasurerarenai is a sad story similar to Socrates in Love, there a both one-shot mangas, too.
Both centers at the problem of loosing something very important, the memories, the hope and the will of living.
There is a high chance that someone who loved one of those two mangas, will love the other too.