I replaced the drive belt because the other one was worn. Before I replaced it the tractor was jerking as well, and now after replacing it it still jerks. The brake pedal goes forward towards me when it jerks.

This sounds like a potential transmission issue. Have you gotten 5+ good years of service out of it with no other major issues?

If so, I'd call a reputable lawn equipment dealer (NOT a big name box-store) in your area who will give it a full "physical" and diagnostic for a reasonable price.
Husqvarna is #2 in my book on riders (2nd only to John Deere) so it's worth keeping alive unless it's on it's death-bed.