I want to think it was only a big mistake

I’ve been with my partner for 18 months and we’ve live together for 6 months. our relationship is great and serious, we don’t have any problems and he has also told me he wants to marry me. But he went out recently and got drunk and blackout and ended up sleeping with another girl. He only confessed me that because after 3 days of having sex I experienced discomfort in my vagina, so I confronted him and he confessed. I don’t know what to do. I want to think it was only a big mistake but also our relationship is so new and we were so good that makes me think of the future. What can I expect when we are together for years and really go through really big problems as a couple. I’m devastated.