If you want "Romantic" or "sad" piano music, many of Chopin's pieces would fit. Some really famous ones are the Ballade in G minor, the Funeral March in the second sonata, and Nocturne in C# minor. Honestly, though, I think Chopin wears a little thin over time- it's too sappy for me. When you say a "few" of Beethoven's sonatas, have you listened to all the movements? You need to, and then I would dig into more of his sonatas. The late Schubert ones are wonderful too (the slow movements of the A major and B-flat major sonatas are gorgeous, though as a whole I think I like the C minor the best). Most of the great Romantic composers (Schubert, Mendelssohn, Chopin, Schumann, Brahms) wrote wonderful piano solos. I remember liking Mendelssohn's Songs Without Words a lot when I first heard them, and you should recognize some of Schumann's children's music. Don't listen to Yiruma. Please.