You need to work on your text game. You can definitely keep things fun and interesting for a long time as long as you are light and witty in your texting.
Every text you send should elicit an emotion in her. Humor is the best way to do that.
There are a few ways you can handle the "what made you think of me?" question:
1. Something random and cute. A line I use often from "The Text Book" is "I just saw the cutest little squirrel outside gathering nuts and it made me think of you

"
2. Something that you and her talked about or did together that could have genuinely made you think of her, but in a funny way. For example, if you two went to Starbucks together you could say something like "I saw a guy driving down the road with a Starbucks cup on top of his roof. I tried to wave him down to let him know, but he gave me the finger and kept going!"
3. Don't answer. Change the subject and don't bring it up again. Keep her wondering.
At any rate, I suggest you look up "The Text Book" or "The Ultimate Guide to Text and Phone Game." Both are excellent sources and have helped me tremendously with texting girls.