Can you recommend a BL manga that follows a couple for many years?

Have you read Kizuna?

There are a few old-school yaoi series that take ages for the couple to consummate ( http://www.mangago.me/read-manga/kusatta_kyoushi_no_houteishiki/ and http://www.mangago.me/read-manga/yatteraneeze/ not to mention the series which starts with this one: http://www.mangago.me/read-manga/mainichi_seiten/), although that tends to be the end-goal and the driving purpose underlying those series. Dear Green is a little more complicated than that. Some of the more recent ones I know (which you've probably read already) are:

http://www.mangago.me/read-manga/kimi_to_parade/

http://www.mangago.me/read-manga/paradise_view/

http://www.mangago.me/read-manga/kyuuso_ha_cheese_no_yume_omiru/

Sequel: http://www.mangago.me/read-manga/sojou_no_koi_wa_nido_haneru/

http://www.mangago.me/read-manga/doukyuusei_nakamura_asumiko/

http://www.mangago.me/read-manga/sotsugyousei/

http://www.mangago.me/read-manga/occupation_to_beloved/

http://www.mangago.me/read-manga/ringo_ni_hachimitsu/

http://www.mangago.me/read-manga/kare_no_barairo_no_jinsei/

http://www.mangago.me/read-manga/kare_no_barairo_no_jinsei/

Then there are the entire Kuroneko, Warui and Mujihi series. And Haru wo Daite Ita of course.

Haru wo Daite Ita,
Hidoku Shinaide,
Bi no Isu series (Bi no Isu, Kobi No Kyoujin, smoke),
Azami series (Azami, Sabita Yoru Demo Koi Wa Sasayaku, Renai-rubi no Tadashii Furikata, Hadakeru Kaibutsu),
and Kimi to Parade series (Kimi to Parade, Paradise View). The last 3 series aren't pleasant or fluffy if you are looking for that. Only Kimi to Parade skips rape or domestic violence.

Thank you everyone!!! Going through all of them!!