All I can remember about the movie is - in the beginning the protagonist is testing some sort of new mecha and it malfunctions and he becomes some sort of monster/cyborg. He ends up fighting against other monsters and he gets some sort of arch enemy who is also fused with a machine. And it is a romance movie, I think the girl he is in love with is the daughter of a police chief?

Sounds like the "Guyver" series from Japan. The hero finds a device that allows him to become a super hero and he battles monsters and the "Dark Guyver" at the end.