i really want to find yaoi manga where the uke and seme already become a couple from the start of the manga. they experienced jealousy and angst all along the plot when others comes between them. hope you guys can help me. thank you.

This one was hard for me being that 99% of the yaoi I have read is about the couple meeting or coming together. I do remember a few where there was already an established relationship...

Aikotoba wa Hallelujah - this is the sequel so they are already in a relationship - someone tries to get in between them
Bousou Kareshi - this is a couple of one shots but the main story is what you are looking for
Tsumasaki ni Kiss
Night Walk
Overdose
Scarlet
24 Jikan Kiss Dekinai

Hopefully, at least one of these will be what you are looking for ^.^

I know a couple...but I'm not so sure the level of angst/jealousy is what you're looking for.

Motto Aishiamasho
Line (OTSUKI Miu) (Oneshot)

The river of life is magnificent and dangerous.

Kizuna - must read.
Bi no Isu
Hidoi Otoko.
Honenuki ni Saretai
Honnouteki ni Otoko
Kanga na Spoon
Lover's Position
Suki Toiunoni Nazekashira
Ubawareru Koto Marugoto Zenbu

There are a number of stories about lovers who grow apart then reconnect, but do not have a continuous relationship. Here's a couple in case you're interested.
Tenshi no Tamagoryouri
Shiranai Kao

Also I highly recommend Haru wo Daite Ita. They become a couple just about right away, and the rest of the long series is about their relationship.

bump 😀

P.B.B. (Play Boy Blues) by Kano Shiuko is not mentioned yet.