¿Esto suena natural? I wonder if I'm not good enough for you.
俺じゃダメかな。（おれじゃだめかな）　

Thank you for the suggestions, dread-spirit!
This question is based on the ranking made in a quiz program. The question is : "What would you like your male boyfriend say to console your feeling when someone dumped you ?" The above answer is ranked second, while your suggestions are ranked third and fourth respectively. “You will find a better guy” is ranked first.

Here are the translations for your suggestions. Either way, I learned new expressions, thank you so much!
'You are too good for him anyway'
君には彼はもったいなさすぎるよ。
' He isn't good enough for you'.
彼そんなにたいしたやつじゃないから。