ML Loves the SO for Years (24+ months), SO Doesn't Know

Types:

1. The ML is by the FL's side for years, but the ML feels that he can't confess his feelings because the FL is dating someone else, the ML is afraid of losing his friendship / relationship with the FL, the ML thinks he's too young / not ready yet, there's a family issue, or the ML doesn't fully realize / accept his feelings. 

1-A. Same as 1 but between two MLs (BL).

2. The ML grows up with the FL but doesn't confess his feelings because he doesn't fully realize or accept them, because he's too young / not ready, because there's a family issue, or because he's afraid of losing or ruining his relationship with the FL. The ML and the FL separate for a few years. When they reunite, it doesn't take long for the ML to start flirting with / trying to woo the FL.

3. The ML and FL were childhood neighbours or classmates. The ML had a crush on the FL but didn't confess his feelings. The ML and FL separate for years. When they reconnect, the ML doesn't recognize the FL immediately and they have an antagonistic relationship or become frenemies. The ML discovers the FL's identity later on.

4. The ML meets the FL briefly when he's a kid and falls for the FL. They separate for many years. When they meet again, the ML recognizes the FL immediately and makes sure to stay by her side, but does not confess his feelings for a long time.

4-A. The ML gets to know the FL for a bit when he's young and falls for the FL. They separate for many years. When they meet again, the ML recognizes the FL immediately and makes sure to stay by her side. He confesses his feelings fairly quickly but does not reveal his identity until much later.

5. The ML is shy. He was the FL's classmate or neighbour and had a crush on her, but never had the confidence or courage to confess his feelings. The ML and FL separate for years. When they reconnect, the ML continues to hide his feelings but develops a closer relationship with the FL.