对一首歌曲的歌词进行分析，并描述歌曲的主题和情感。

提供《Yesterday》这首歌的歌词。
歌曲：Yesterday
演唱：The Beatles

歌词：
Yesterday, all my troubles seemed so far away
Now it looks as though they're here to stay
Oh, I believe in yesterday

Suddenly, I'm not half the man I used to be
There's a shadow hanging over me
Oh, yesterday came suddenly

Why she had to go, I don't know, she wouldn't say
I said something wrong, now I long for yesterday

Yesterday, love was such an easy game to play
Now I need a place to hide away
Oh, I believe in yesterday

Why she had to go, I don't know, she wouldn't say
I said something wrong, now I long for yesterday

Yesterday, love was such an easy game to play
Now I need a place to hide away
Oh, I believe in yesterday


主题：失去与怀念。歌曲描述了一个人失去了曾经珍视的爱情，因此感到无助和痛苦。歌曲中反复提到“昨天”这个词，强调主人公渴望恢复过去的生活状态。

情感：伤感。歌曲的情感表现出主人公对过去美好生活的怀念，以及由于失去心爱的人而带来的愧疚和痛苦。