I caught girlfriend canoodling and it's ruined all my plans

Recently, I was at a house party with my girlfriend and walked into the kitchen to see her in a clinch with another guy. It was obvious something was going on between them or it was the start of something going on.

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I lost my rag and pulled him away, shouting. It’s not my proudest moment, but I was upset and just saw red. She was in tears and I left soon afterwards. It was really humiliating.

We spoke the next day and she apologised, and I said sorry for shouting.

She said nothing happened, but I saw something different. We're both 28 and have been together for four years, but live an hour's drive from each other, which isn't ideal.

She's obviously not as committed to the relationship as I am and I'm wondering where it leaves us.