My girl is my world how do I show her that?

You know you sound like you are doing pretty good and even though distance is hard to deal with its great you want to make the effort. You can show you care with small things like the next time you are thinking about her text her or call her and let her know that will make her feel really good. Next time you talk to her ask her what's going on in her life and ask about stuff she told you about the last time you talked this will make her feel like you are more involved in her life.

Call her every day, if there are days where you are super busy call her before going to bed just to say goodnight and how much you love her, wish she were there with you (in bed lmao jk). See the conversations don't necessarily always have to be 3 hours long, sometimes even 5 minutes shows you are thinking of her. Text her when you can something sweet. Share your experiences so she'll feel that even though she's not physically with you at the moment she's still there with you. Never lie, never cheat, always respect her. Listen to her, sharing experiences is mutual. Plan something fun for when vacation comes. Plan a weekend away together when you both have time to get away, I know how busy things get with school but spring break is coming soon, that's a great opportunity to have alone time together and have fun.

Actually that's a really hard situation but if you really love then tell her what you truly feel.
Look at hers and say it to her promise her that she can trust you. and of course you have to keep that promise..
and you have to say it from your heart.
or tell her, text her that you love her...