One of my greatest fears of songwriting is having all my songs sound the same. As a self taught player who hasn't studied any music theory (starting to now) I can share from experience.
Unfortunately there is no magic formula to writing songs. Something I recently tried doing was writing without an instrument in my hand. But lets talk about what happens when you try to write a song with your guitar.
Every time you sit down to write something by playing you aren't necessarily creating anything new. What limits your creativity is your playing ability or habits (this can go for all players). Over time you pick up habits of playing that same shape or scale, or perhaps you really like this one lick that is very easy for you to play. When you are writing music with your guitar in hand, you're only writing what you can play. With this you aren't going to advance your songs technically nor are going to have very different sounding pieces. I soon realized this with my song writing after some feedback on my songs sounding "boring because they all sound the same" after running out of ideas.
However you can fix this. Writing without a guitar in hand, you are able to break the walls down and expand your pieces of music. Now I don't mean this by sitting down at a desk and writing out scores of music from your head. I mean this by using software such as a MIDI editor. Most of us here who have downloaded guitar tabs know what a MIDI editor or more easier put, tabbing software. You can find all sorts of tabbing or midi software on the internet with a Google search. Using software such as Guitar Pro or Power Tab to write your own pieces of music allows you to write what you want (rather than being limited by skill and/or habits of playing). Editors have lots of features that can help assist in the songwriting process. Learning these features can really help make it much easier and as least painstaking as possible.
Once you write your song you basically have to learn your own song. If it's challenging to you than you know that what you wrote would most likely not be anything you'd write if you had tried writing by playing. Another advantage to writing your songs with tab software is that you are able to print out your tabs in nicely formatted and customized sheets or go back and make some changes whenever you feel.
As I began writing more and more complex pieces the more I realize how I wish I started studying music theory earlier on. Theory goes a long way and I've barely just started realizing how useful it can be. But by knowing theory you can speed up the song writing processing greatly.
Anyway I think that sums it up. If you're having a tough time creating different sounding recordings than try writing some songs with some tabbing software and see how it works for you. I highly recommend learning all the tabbing software's features so you can get the most out of it. Hope this goes over well, it's my first article here! Thanks!