'Itooshiki Hibi'-- the age gaps a bit much (it's not a minor and adult thing just the age gap) but it's a wholesome story otherwise
'Will you marry me again if you are reborn'-- this one fits your description, really good story from start to finish
life senjou no bokura. they're dating and there's a time when they broke up but they eventually got back together and spend the old days together
Taming the Tiger
I just finished reading it today fuck im crying so much
Story where they meet when they are young and they became each other lover until they grow old