There are people who will support us in the world and others who will "attempt" to exploit our vulnerabilities for their own advantage or convenience, and this is not always a conscious act. Some people will defend against recognizing their own tendency to exploit others. It could be that your ex sometimes believed his own deceptions, including the gaslighting.

The important thing is to recognize where you are vulnerable, so you can build boundaries around it and protect yourself, while you work on becoming less vulnerable.