Pyotr Ilyich Tchaikovsky's "6 Romances" Op. 73, No. 5 ('None but the lonely heart') is one of his most beloved songs, renowned for its intense emotional depth. The piece is distinguished by its sweeping, yearning melody and rich, chromatic harmonies.
The primary technical challenge is to infuse the melody with profound pathos and maintain its expressive line through sensitive rubato and pedaling.
6 Romances (Op.73) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7. In the RCM system it corresponds to about level 8.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 6 Romances (Op.73) lasts about 5 minutes.
6 Romances (Op.73) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.