Is there a specific reason you changed to using cookies?
In reality, setting the cart to be cookie based is not a great idea, there a re a number of limitations to using a cookie based cart. The biggest limitation to using a cookie based cart is that cookies have a maximum size. You will only be able to add so many items to the cart before the maximum cookie size is reached.
This maximum size is different for each brow3ser, so there is no real way to anticipate when that size is reached.
Officially, the limits of using cookies are:
* 4 KB per cookie maximum
* 300 total cookies, for a total of 1.2 Mbytes maximum
* 20 cookies accepted from a particular server or domain