I can't understand this どれ程{ほど}の人間{にんげん} part well:
その人{ひと}の中{なか}で、私{わたし}はどれ程の人間になっているというのか。
Is it something like:
What kind of person do they think I am?
どれ程の人間 in this sentence means どれ程重要な人間. The sentence means "I wonder how important I am to the person."