Something similar with "My Mister" but with romance! Any recommendation?

Both of them! But I prefer couple on their 40s or more… or only the woman younger! The import part is the ML being gently and reserved! Thank youuuu! 

Only series I can think of where the ML is in 40's. Not many with more older than that really.

Chocolate    (ML 41 at the time, but not much age difference between ML and FL)

Dong Yi    (ML 39 at the time)

Prime Minister and I    (ML 43 at the time)

 Yumi:
The import part is the ML being gently and reserved!

Romance dramas with gentle and reserved MLs in their mid 30s:

 Yumi:
I prefer couple on their 40s or more

Romance dramas with MLs in their 40s that are sometimes reserved, but not always, and that are not really that gentle generally, but they develop a soft spot for / fall hard for the FL: