She couldn't believe she finally had one.
What is it that she finally had one of?
She was his senior, and she told me that she had been wanting to get with him since junior year.
It can be a real joy to see hard work pay off and land a relationship. Relationships can be enjoyable and might lead to long lasting ones.
She acted naive and even made a mistake, thinking she was pregnant.
It's great that she was able to appreciate the new relationship. It's important to not take other people for granted.