My husbands ex has contacted him over Facebook?

We've been married 2 years. Together 6. He got with me pretty quick after leaving her. He was in a bad place they were together 5 years and she hurt him. He was living in London & because he was a student in university to transfer to one in Wales to get away from London and start fresh. That's how we met. Months after getting together with him (wrong of me I know) but I had feelings he was talking to her so I went though his Facebook and phone. Whilst being drunk he's sending her texts. This was 6 years ago. We are married and have a son. Live together in London. But last year he was on his iPad next to me on Facebook, and a notification popped up which made a sound and it was a message which he opened not expecting it to be his ex, and I saw. She said how is he and she misses him. He didn't reply (not at that moment) & we kind of had a little argument. He said he's completely over her and doesn't care anymore. Well a month ago I went into his contacts and searched through and came across a name saved as the same name as her. I rang it off mine on private and I heard her voice when she picked up. I hung up. And now today whilst he's at work I again been on his messages on Facebook and they've been talking behind my back! I haven't mentioned any of this to him but tonight when he comes back there will be hell. He's not over her is be? They've been talking friendly but still he's been lying to me. Should I divorce him? I can't be bothered for all this and getting hurt.

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