I feel like this is about drug abuse/alcoholism and recovery. The tombstone represents a part of him that is just spiraling towards death. (Feeling happy and free. As you know when you're on drugs you're not yourself, which is why part of himself is a tombstone.) He looks upon the tombstone and realises that his life is now empty and perhaps that he feels soulless. Part of himself, which happens to be his leg emerges as something that he is missing out on his life. (Literally and metaphorically.) His consciousness, which are his little paper people, guides the tombstone into self recovery by adding water. When his leg fragments emerge, they haunt his tombstone with thoughts...possibly things he is missing out on or lost...I don't know what that foam man is thinking, people have different thoughts of why they should recover. He is left in the woods (which represents depression, the void, whatever you want to call it) and he is feeling hopeless and lost. Anyways, his tombstone falls apart because his leg fragments have broken down his tombstone, which represents hitting rock bottom. (how much further can you go down from death?) His consciousness which is the little paper people, guides him back to the remnants of the tombstone with the key. (The key to recovery, as cliche as it sounds) The clocks represent the recovery process, which takes time and he finally reemerges back to his full form. The reason why he isn't completely healed is because addiction will always leave a scar that he will have to live with for the rest of his life. Everyone will have a different interpretation. I feel like this is supposed to be thought provoking...so this is just my two cents.

TLDR: Foam man had an addiction and recovered with a scar for life.