He knows Minato is insecure about the boss but still went to cafe and shopping with him even had him drive him to his date. He fears that Minato might leave him like his ex-gf did but still doesn't do anything to change
Right!! He's so inconsiderate of Minato's feelings, like this relationship feels like Minato is the only one putting in effort. Instead of fixing the issue by putting up boundaries, changing, be more assertive and prioritize Minato's feelings, he goes on these weird "dates" with his boss. Imagine having so little time to spend with your boyfriend because he's busy, but he chooses to spend some of that with his fucking boss when he knows how that makes you feel, I would be livid and question where tf do I stand on his list of priorities? Doesn't seem like I'm up there because my partner is a pushover and people-pleaser. If Shizuma doesn't see the problem he's causing by worsening Minato's already exisiting insecurities, then he's just incredibly obtuse and he should not be surprised when Minato reaches his breaking point because enough is enough. Actually, Minato is better than me because I would've called it quits already.
Shizuma is a pushover with no backbone. "Being too nice" is a problem, clearly. He doesn't know how to priortize his own lover's needs over others. Sure, be nice but learn how to be more assertive?? No backbone whatsoever. Obviously it's tearing your relationship apart?? DO SOMETHING MAYBE? To have someone like that as a lover, that's exhausting but somehow Minato's the problem? Surprised Minato hasn't broken it off with Shizuma yet because atp, it needs to happen because clearly, no amount of communication solves the issue. There's some self-reflection needed.