Why did he propose but still cheats on me?

My fiance cheated on me several years ago and said he broke up with the other woman already. He proposed to me, tells everyone at work, his friends and family that we will get marry soon. I found out that he is still cheating on me now but he doesn't know I am aware of it. I am so confused. Why does he seem like he is ready to make a commitment only to be still cheating with that same person he met years ago? I know she is a single woman so why wouldn't he just end our relationship and start a legitimate relationship with her? Why did he propose to me and he even seems like he is more impatient about getting marry than I am but continues to see the other woman for years. Tweet