Shounen No Kyoukai
It started off with Yuka and Kaoru who's childhood friends but once they hit middle school Yuka distanced himself from Kaoru and befriended Taiga. Once the second gender exam came and Yuka finding out he's an omega, everything took a turn. Especially when he found out Taiga is an alpha and so does Kaoru. Yuka who suddenly went into heat and almost getting rape by Taiga was forced to be pair with Kaoru who believes they're fated pair. [S1] Taiga who said omegas are just for breeding during middle school is now in kind of a relationship with Rin. Helping him with his heats and letting him live with him. There wasn't much difference between Kaoru and Yuka too. It's just that they act more like friends to each other except when Yuka's in heat. One day, Rin met Kaoru his destined fated pair. Rin who was desperate to be Kaoru's pair but was rejected by Kaoru who believes his FP is Yuka got overdosed on supressants meds saying he'll wait till Kaoru and Yuka breakup and accept him. Taiga makes Rin realized that all this time he loves Rin and they became a pair. But that wasn't the happy ending. Yuka got pregnant but Rin began to slowly lose his memories as the side effects of overdosing on pills. It got worse actually. Until he forgets he's paired with Taiga and even until he forgets who Taiga is. But they both recreate their memories again.
Wagamama mo Kimi no Tame Nara Ikura Demo
No More Lonely Night