I just got back from my first solo trip and I LOVED it! There were a few times I wished I'd been with someone, but far more times when I was actually glad I was on my own. I got to skip the things I was never interested in but the rest of my family always wanted to spend time on, and do things like go on Everest seven times in a row without people getting tired of waiting on me. I got to eat when I was hungry and not when most of my group was, it was up to me if I got up early or slept in (made it to Rope Drop for the first time ever, something that would never have happened with my family there). Sometimes I'd just be wandering around and see street singers at HS or a character I like at MK, and I could just stop and take my time without being hurried-up.
I had a loose itinerary, but not the minute-by-minute ones that some people have (like we used to have when I went there as a kid!). I planned like "MK until around 3pm, then Epcot thru evening, Illuminations then home". Once or twice though, it'd be time to head to AK or whatever and I'd be like, "yknow what, I really would rather go to HS - let's do it!"
One thing I'd suggest is look at the park maps (you can download them in various places including the official WDW site, I think) and write down all the attractions/rides you'd like to do. Pay special attention to things you've always seen on the maps and never gotten a chance to do because no-one else wanted to. My family had always skipped Flights Of Wonder at AK so I put it on my list as a "must do" because I really wanted the trip to be all about MY to-do's.
It does feel a little awkward for the first day or two saying "one" when you're asked how many in your party, but I was surprised both at how much no-one else in groups gave me a second look, and how nice other people were. I got talking to another solo traveler in the Dole Whip line, had a CM sit down and tell me all about the building of the Ballroom when I was eating at BOG, swapped stories with a solo traveler at the resort bar, and ate and chatted with three different families who were super friendly when I asked if I could share their (big) table in busy restaurants. It was great to have some friendly interaction with other Disney people, and then wander off back to do my own thing at the end of it. And I'm a very shy, quiet kind of person when I'm alone, so if I can make randomly make friends in the parks, anyone can!
You'll love it
Just to add: Pick a few things that you'd love to do there if you were with someone, and challenge yourself to do them anyway even though you'll be alone. I really wanted to eat at BOG but the thought of sitting in a restaurant with families all around me made me really nervous. I made myself do it and got in the line for lunch, and it was one of the highlights of the whole trip. And it was a great step up to doing TS meals by myself, which I've made a few ADR's for during my May trip!
Since I was the only one who had already bought the plane ticket I figure ok I can do this...
