As a Frenchman expressing your feelings to someone special, you can use some of the following romantic words and phrases:
Je t'aime: I love you (the most straightforward and classic way to express love).
Mon amour: My love.
Ma chérie / Mon chéri: My darling.
Je suis fou/folle de toi: I'm crazy about you.
Tu es l'amour de ma vie: You are the love of my life.
Je t'adore: I adore you.
Je pense à toi tout le temps: I think about you all the time.
Tu me manques: I miss you.
Tu es magnifique: You are beautiful/gorgeous.
Je ne peux pas vivre sans toi: I can't live without you.
Tu fais battre mon cœur: You make my heart beat.
Je suis tellement chanceux/chanceuse de t'avoir: I'm so lucky to have you.
Je rêve de toi la nuit: I dream of you at night.
Tes yeux sont sublimes: Your eyes are stunning.
Tu es la plus belle chose qui me soit arrivée: You are the best thing that ever happened to me.
Remember that expressing feelings in a romantic context is all about sincerity and being genuine. Speak from your heart, and use these words or phrases to complement your emotions and affection for that special person.