If I sell the patent I applied for to others, can I still use it?

It depends on what is stipulated in the contract you signed. Under normal circumstances, if you transfer all the rights of the patent to the other party, the patent has nothing to do with you, and it is up to the other party to decide whether it can be used. If your contract stipulates that only the other party can use the patent and there is no restriction on others to use it, then you can use it.