My boyfriend may have cheated on me while he was intoxicated.

My boyfriend (M24) told me that he might have cheated on me while he was at a rave on Saturday night. He mentioned there's a 50/50 chance he did because he was under the influence of drugs and “wasn’t himself.” He said his friend gave him what he thought was cocaine, but it ended up being ketamine. He claims he can't remember anything from that night and wants me to forgive him if he did cheat, insisting he would never do something like that when sober. I'm feeling completely lost. We've been together for over five and a half years, and I envisioned a future with him—marriage and kids. Now, I’m grappling with feelings of betrayal, anger, and devastation. My greatest fear has always been being cheated on without knowing it, and he knows this all too well. My previous boyfriend was unfaithful, and now I feel like I'm stuck in a nightmare. I can't shake the thought of him being with someone else while he was with me, and it makes me feel sick. He keeps suggesting I shouldn't be this upset because he wasn't in the right state of mind, but he still made the choice to go out and get into that situation. Am I being unreasonable or overly dramatic for considering ending things over this? It feels like a deep betrayal. I’m uncertain about how to proceed and would appreciate any advice.