You can’t stop people from using a false From: address such as yours. It’s like trying to stop someone from putting your return address on an envelope in US Mail. It’s happening outside of your reach. In this case, it’s a common tactic, so you’re not the only one with this problem.
It’s also not uncommon for spammers to set the From: to be the same as the To: in order to try to get around your spam filter.