Be careful about what your motives are for telling him. I don’t actually believe ‘honesty is the best policy’ applies in these situations.
Often people ‘vomit’ news like this into their partner’s lap in an attempt to make themselves feel better. How fair or caring is that to the partner? How respectful to your relationship? There doesn’t seem to be any pressing external need to let him know- no pregnancy, STD, no chance of the lover coming forward? Then maybe deal with your guilt on your own (or with a counsellor).
Also maybe examine this overwhelming guilt thing. Have you bought into some religious or cultural idea that you can’t/shouldn’t have sex unless it’s with the love of your life? Do you believe that couples must absolutely share every single thing that ever happens to them with each other, otherwise it isn’t ‘true love’? Do you think you’ve somehow tainted your relationship and given up any right to fidelity from him into the future?
All these are possible, but not really feasible policies to take into modern relationships, surely? Sounds to me (I may well be older than you) like the kind of thinking someone does when they are a bit immature, a bit of a drama-llama or a bit indoctrinated by a ‘reality tv’ idea of romance. Sorry, if that’s harsh.
Your choices aren’t just ‘tell or don’t tell’. You can also unpack and process this guilt on your own, stop building it into the Secret Of The Ages, and forget about it altogether. Only one person is giving this other guy importance and relevance in your forward relationship. Seems a lot of power to give a fairly random one-night-stand.
Put it behind you, don’t look back.