Based on reading this, I tried to convey what is expressed in your description of how you felt about the city! So, I tried to write more like poetry or lyrical. Don't pick up immediately from these, yet though, but have some thought about them, or have other natives read it, and go from there since it's permanent engraving on your skin. But, these are what splashed in my head as I was reading your description with my heart pounding because I could feel how much you were in love with Seoul! It's a city that means a lot to me as well!
i was in seoul in 2015 and i loved it so much. so much so, that i feel like a piece of me was left behind and will forever be with the beautiful city!...
- 내 영혼아 서울과 영원히 : My Soul! Forever With Seoul
- 내 영혼은 서울과 영원히! : My Soul, Forever With Seoul!
- 내 영혼은 서울과 함께・・・: My Soul, together with you, Seoul・・・
- 서울이여 나와 함께 영원히! : Seoul, thee forever with me
- 서울이여 영원히 나와 함께 : Seoul, forever I'm with thee
- 서울이여 나하고 영원히 : Seoul, with me forever
- 내 마음은 서울과 영원히 : My heart, forever with Seoul
- 서울이여 나와 영원히! : Seoul, forever with me!
- 아름다운 서울이여 나와 함께 영원히・・・: The beautiful thee, Seoul, Forever With Me
- 내 마음은 영원히 서울과 있으리라 : My heart will be with Seoul forever
- 내 마음은 언제나 서울과 함께・・・ : My heart will always be with Seoul
These writings can be read as energetic, melancholy, or even sad depending on where you place punctuation marks (if you care) such as「！」,「・・・」, and 「～」
- 내 영혼아, 서울과 영원히！ : My Soul! Forever With Seoul!
- 서울이여 나와 함께 영원히・・・ : Seoul, thee forever with me
- 서울이여! 영원히 나와 함께・・・ : Seoul, forever I'm with thee
- 서울이여 나하고 영원히・・・ : Seoul, with me forever
- 내 마음은 서울과 영원히・・・ : My heart, forever with Seoul
- 서울이여 나와 영원히・・・ : Seoul, forever with me!
(9) ,(10), and (11) are my personal picks (the one in bold) despite of its length because its number of syllables closely matches to that of ancient Korean poetry frame. At least they sound quite poetic and rhythmic to my native ear while the others sound a little more like calling-out-like. As for (9), I like the rhymes coming from the series of similar syllables from 름/운/울, followed by an emphatic sound 여,
and 나/와 which all together forms a flow from 'down/neutral' to 'open/out-casting' and eventually 'rising but fainting' sound of '히'. It even feels melodic to read it. (10) sounds good mostly because the 있으리라: classy and emphatic. (11) sounds just plain modern but omitting the verb makes the phrase sound good and add the feeling of longing for the city.
It doesn't explicitly say 'you left the city behind', but, you can leave it to be inferred by readers; and when someone asks about it, you can give them the full story along with a cup of tea; maybe they can come to tears after they realize what it means :) Isn't that the beauty of East Asian poetry--omission?