Even if he dissolve the engagement with her her, that doesn't mean he can easily marry his lover who is a commoner
That doesn't make any sense
The only reason I could think of is that he doesn't want to be the one to dissolve it? But he could have ask her to dissolve it instead. Either because the lover convinced him to, he thought she won't agree, or the king didn't allow it for such reason.
He looks so much like a coward and for him to do such thing to his ex fiance... Either he never was or will be a good royalty or his supposed character development never happened because the original story didn't follow through.