It's not like jimin didn't like his girlfriend. He thought she was pretty, sweet, nice and over all a great girl. She was very religious but he found that cute. He knew he wasn't bisexual but he still felt something towards the girl, he later found out that at it wasn't romantically and used her to cover up him liking other males. He didn't want to end things because they've been together since 8th grade and she was already whipped for him. He cared about her and didn't want to see her hurt. So he stayed with her. Suffering, guilty,sad, and afraid every. single. day. He always found a date to break it off but on the same day she'd buy him something or ask to go out on a date so it was hard for him. But he knew he had to do it soon because the new kid already gave him butterflies.
