Battle through the Heaven or just look for CN ones with broken engagement and reincarnated/transmigrated tags
But do you want the ex-girlfriend/fiance to get back together with the MC or just regret it? Agreed on BTTH
That's like a stereotypical generic xianxia trope, so just pick a random one and there's a good chance it's in there
Dominating Sword Immortal is another one ... tbh ... I think way too many of the Chinese novel I (tried) to enjoy have this ... as an element of storytelling ... IGE - Immortal/Imperial God Emperor (me thinks) has a similar trope - not a girlfriend, but his childhood friend (female)
Spoiler: Lan Yunyue Lan Yunyue, she leaves the MC to join a Young Master for power. The MC in the past didn't had any good future as cultivator, but later he became really strong. Lan Yunyue became the fiancee of the Young Master for power and status but she still had the MC in her heart, the young master was angry for that and tried to kill the MC, he died, Lan Yunyue regrets everything.
Thanks! Sounds stupid and you forgot to put a spoiler warning. I dont care, but NU will want your head for it.
I can already see some random jackass strolling through the comments and seeing yours. Then he will squeal like a pig and start reporting your ass. Its happened to me more than once.