▲查了一下这一句的上下文，是这样的：

I will also be happy because I will be an adult then. It’s hard to focus on being happy when you are a teenager! We have to work so hard at school, and when we try to take time to relax, there is always someone telling us things we should be doing or learning. In the future, I will be more mature and be able to make my own decisions and do the things that I enjoy. I will have a secure job that I like and I will find happiness in being successful at it.

▲网上也有翻译：

我感到幸福还因为那时我已是成人。青少年时期，你很难专注于幸福!我们在学校里必须刻苦学习，当我们试图想抽空放松一下时，总有人告诉你应该做这个、应该学那个。到了将来，我会更加成熟，能够自己做决定，做自己喜欢做的事情。我会有—份自己喜欢的稳定工作，我会因为工作成功而找到幸福。

▲根据句子的上下文来看：这里的 in being successful at it 可以换成 for being successful at it。用 in being successful at it，强调过程，指在成功的过程中；用 for being successful at it 强调原因，指因为成功这个原因。但不宜换成 to be successful at it，因为不定式既不表示过程，也不表示原因。