Time is something that we cannot go back on. There will only be one "today." Today will never happen ever again (unless you believe in multiverse theory or a universe way vast than our own). But the character realises that he can never go back to the "good times" and relive them.
He is barely making it out of an extensive argument with I presumable another person... or it could be his own life in general that he is referring to, or both, as usual.
The character tries to recall good memories he has had in order to keep himself happy. He is, presumably, lonely now and wishes to reminisce on better times in his life.
The character is beginning to forget his memories as he begins to get older. Either he is intentionally forgetting bad times or he may have a condition that makes him forget memories without choice... Or simply it is because he's getting older and people tend to forget as they grow old.