My best friend of 8 years lied to me for 1.5 years about a girl and I don't know if I'm overreacting

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I'm a 25-year-old guy and I've been best friends with this person for around 8 years.

About 1.5 years ago, he became close to a girl. Over the years, I asked him multiple times whether he liked her or was involved with her in any way. Every single time, he told me no and would say things like, "She doesn't fit into my life like that" or that they were just good friends.

The issue is that I had a direct altercation with this girl at one point. She belittled and disrespected me, and my friend knew exactly how I felt about it. After that incident, he told me he would confront her about her behavior.

I recently found out that not only were they more than friends, they had been in a physical relationship. When I confronted him about all the times he denied liking her, his response was basically, "I was being dishonest with you about this."

What hurts me isn't that he was involved with her. He's free to date or sleep with whoever he wants. What bothers me is that:

1. He lied to me repeatedly for a long period of time.

2. He knew this girl had treated me poorly.

3. He told me he would confront her, but instead continued the relationship.

4. I considered him my closest friend and told him everything about my life, relationships, and career setbacks, while he hid something this significant from me.

Right now I've decided to stop talking to him for the foreseeable future. I'm not blocking him, but I don't really see him as a friend at the moment.

My question is: am I justified in feeling betrayed here, or am I taking this too personally because of the history and emotions involved?

I'd appreciate honest opinions, even if you think I'm wrong.

Thanks in advance!