我觉得,这句话可能口语化点会比较好.应该深情点哈.
maybe I'm not the best (one),I'll get better for you.I'll always persist,because of you.
这是很口语的说法,这里的“但”不适合用but,因为,用了maybe足以表达后面的潜在转折.
或许我不是最好的,但我会为你变得更好,我会为了你一直坚持下去.