My boyfriend of 4 years has a drinking problem and cheated on me with his ex while out drinking. I am a non drinker so I do not hang out with him at bars. His ex started to reach out to him 2 yrs after our relationship started and he wouldn’t reply to her at first and told me about it but then she texted him about her daughter having cancer and he replied. That opened the door for them to start conversations. Since I found out about him cheating, he quit drinking and stopped all contacts with her. Should I forgive and stay with him?