Consider the following to retrieve bitcoin from the scammer
-cut off all contact immediately
-check the transaction where your bitcoin went
-report the scam to your local police or file a report (but do not expect that much help from them as you know the nature of bitcoin)
-if you used an exchange, tell them right away, they might help freeze the funds
-share your experience on bitcoin forums or other social media to alert others
-keep an eye on your wallet and accounts for any strange activity

Unfortunately, once bitcoin is sent, it is hard to get it back, so always be careful before sending any funds