Try this. Sit down with a guitar and write a chorus around the chords E - C#m - A - B. Just voice and guitar. Spend twenty minutes on it.
Then go to your DAW, load up a pad synth, and write a chorus around the same four chords. E - C#m - A - B. Twenty minutes.
You will not write the same song twice. You will barely write related songs. The melody will move differently. The phrasing will land in different places. The implied tempo will be different. The harmonic feel, despite being the same chords, will resolve to different emotional weight.
I do this exercise sometimes when I'm stuck, and it never stops being weird how much the starting instrument is doing.
Why the instrument leaks in
Every instrument has physical constraints that bend the music in a specific direction whether you mean to be bent or not.
A guitar is rhythmic by default. Your strumming hand is a drummer that's always playing, and the pulse of that hand is sitting underneath every melody you write at the instrument. Even fingerpicked, there's a recurring pattern your fingers fall into. The melody learns to sit on top of that pulse. The phrasing learns to land on the strums you can actually hit. You unconsciously write topline notes that the open strings support.
A piano is harmonic by default. Both hands work, the left hand can do anything, the chord voicings can be voicings, not just shapes. Melodies written at a piano tend to move more freely because the harmony underneath is doing more work. The voice has more room to wander because the chords are richer.
A DAW with a synth pad is a pillow. There's no pulse, no shape, no constraint. You can hold a chord forever. The melody you write on top of a sustained pad has to invent its own rhythm from scratch, because nothing underneath is suggesting one. Often this is why DAW-written melodies feel a little drifty until production rescues them.
None of these are bad. They're just different. And whichever one you started on is already inside the bones of the song.
The DAW trap
If you start every song in the DAW, your songs all carry the DAW's particular weather. They tend to be built around loops because the DAW makes looping the easiest move. They tend to live in keys that match the sample packs you reach for. The arrangements tend to be additive -start sparse, layer up - because that's how the timeline feels when you scroll right.
This is fine, until you notice every song you write has the same arc and the same emotional ceiling. That ceiling is the DAW's, not yours. You inherited it the moment you opened the project.
The guitar trap
Guitar players have the same problem in reverse. Almost every song you write at a guitar will fall into the keys that sit well on the instrument - E, A, D, G, C, sometimes B minor or F#m if you're feeling spicy. The strumming patterns will cluster. The bridges will tend to switch to the relative minor because that's what guitar bridges do. The choruses will land on the open-string chord that rings the loudest, because your hand wants it to ring.
If you've only ever written at a guitar, your catalog has a sonic shape you didn't choose. You chose songs. The guitar chose the rest.
Use the leak on purpose
Once you know this is happening, you can use it.
If a song you started in the DAW feels stuck, take it to the guitar. Just the chords and the melody. Spend an hour with it acoustic. You will hear which parts of your DAW arrangement were doing emotional work and which parts were filler that the loop happened to support. You will also discover melodic moves you wouldn't have found inside the DAW, because the guitar's pulse is suggesting them.
If a song you started on guitar feels stuck, do the opposite. Open the DAW. Don't try to produce it. Just put the chords on a pad and improvise a melody over the static harmony. You'll find topline lines that aren't tethered to your strumming hand, and some of them will be better than the ones you wrote at the guitar.
This isn't a trick to finish the song. It's a trick to hear it. Once you've heard it through two different instruments, you know which parts are load-bearing and which parts were the instrument talking instead of you.
If you only have one starting instrument
You don't need to learn a new instrument. You need to introduce a constraint that breaks the default leak.
A few that work:
Capo the guitar in a key you hate. You won't fall into the open chord shapes because you can't. Your hand has to invent something.
Open the DAW and forbid yourself from looping for the first hour. Linear writing only. You're forced to develop ideas instead of repeating them.
Sing the song into your phone with no instrument at all. Voice-only writing exposes the melody and lyric with nothing else to hide behind. Then bring an instrument in second and let the voice memo lead the chord choices, instead of the other way around.
Detune the synth, or write in a tempo you never use. Same principle. Anything that breaks the muscle memory the instrument has built up in you.
What this is really about
Production gets blamed for sameness in a catalog. Mixing gets blamed. Plugin choice gets blamed. They all matter at the margins, but they're all downstream of one decision that happened before you opened the project: what was in your hands when the song started.
If every song starts the same way, every song carries the same fingerprint. Change the starting instrument once in a while and you'll notice your songs developing parts you didn't know they had in them.
The song doesn't really care which instrument you use. But it always remembers which one was there first.