you normally need decent grades or a good excuse to be allowed to overload.
like ace said, don't overload if you're just making up for units that you've failed. you will do worse. don't say you're going to try harder or whatever bullshit next semester - it isn't gonna happen, and you'll just do worse because you suddenly have more units than you can handle next semester.
the summer course is the best option... summer courses aren't all that bad, they're usually only a few days per week (depends on the course of course)
of course you can always overload and prove me wrong.