So much music has been made, and is readily available, it's pretty hard to come up with something completely new.. Write what comes out.. don't worry about 'being totally original'... cause you really can't.. The music part of our brain, is full of data, rules, what sounds good, scales, melodic forms it likes etc.. It takes all that's stored in it, and something comes out. Sometimes, it feels like music comes 'thru' us and we are only the conduit..
When Paul McCartney first wrote 'Yesterday' (it was called Scrambled Eggs). He said the melody came to him as a fragment from a dream.. He walked around for a week or two, playing it for everyone he knew, being pretty sure at the time, it was something he heard from someone else, and not totally his own. When he was satisfied no one knew what it was, then he felt it was his own..
After you've got a concept out with your creative 'subjective' brain.. Be analytical about it, did you steal something.. Or are you just using a compositional tool, other composers have used.. Sometimes I just jam a melodic part out.. Than cut/paste/move segments around, until it sounds unique, and not like a melody I would commonly write..
Being 67 and very versed in popular music (I took music lessons from 30's - 60's fake books).. I can hear so much borrowing, stealing, manipulation of something old, outright copyright infringement. I sometimes think if I was a music lawyer, I could make a living on music copyright infringement alone.. A lot of cable stations, cater their commercials to cities, even segments of the population, so a fair amount of these commercials are 'home made' by younger musicians.. I've heard popular music songs, just barely altered and used as original music. (I used to compose for OmniMusic Library). Back in the 90's I had to do 16 songs in 3 weeks.. I bought and used Band in a box.. to speed up the process. It's a clever piece of software.. But once you use it enough you start to recognize chord progressions and melodic segments it uses. Imagine my surprise to hear a moderate amount of generic music used in commercials, industrial soundtracks, that I knew the original ideas came from Band in a Box..
Write what you like.. if you feel like you 'regurgitating' someone else's music it's OK.. You got to get it out of your system, then sub-consciously you will move on.. Since so much (all?) classical music is public domain, don't worry about it.
good luck, keep making music.