I'm Lonely but my Friend Needs Space

I met this girl in school, and I thought she was really nice. As time went by, I developed feelings for her and I confessed to her one day. She said she wasn't ready for a relationship, but we've still been friends since. I still had a lot of questions, but I didn't want to bring it up again because I really liked her and didn't want to lose her as friend by being too pushy. At the end of the school year, I decided to write her a letter asking her how she felt about me. However, I was too nervous to give it to her. We were always a little awkward around each other, and we mostly texted. But we still used to have really great conversations. Now, she only answers with a couple words and leaves the conversation really quickly. She isn't always on her phone, but I promised not to bother her about not texting back because it's one of the few things I do that seem to irritate her. I feel like she might need some space, but I feel so lonely without her. She's also going on vacation sometime this summer, so I won't know if we'll be able to keep in touch.