What if you say things you don't mean to say when you are upset? how can you apologize?

i found some old xrated pictures of my husband and his old flame. i was very upset and told him about them. he told me to delete them he had forgotten they were there. (he had given his laptop to his family, so he had not used this laptop for some time now). well he was totally innocent and i believe him, but i was so enraged that i proceeded to bring up old stuff that i never told him about me...why? i don't know...i had a sharp tongue and he was doing his best to calm me down and apologize. i was def. hurt, mad, sad...etc...i elluded to going out with a guy from his college that i never told him about. he ignored this and said that it didn't matter...nothing matters but the present...so we made up. but i yoyoed a few times over the weekend. feeling good some time, then feeling lousy others and crying just thinking about the images i saw....he did his best to be there and listen to me. well faster forward a day and we are hanging out with our daughter. i tell him that things are still on my mind and and he acknowledges. we are there playing with our daughter then he asks to revisit the things i blurted out.so he asks what i was refering to when i mentioned that other guy. well what i had never told my husband is that the day we met i met this other guy. it was a party. so i went out with my husband a few times and i went out with this other guy once after i went out with my husband. well dinner and drinks turned to hanging out at his place...so we had sex. after this...we talked briefly but i was not interested and i told him that i was now involved with someone seriously so we could not go out any more. so when my husband asked about this...i asked if he was sure he wanted to talk about it and he said yes. so i was honest and told him about this. now i would have never mentioned this if i hadn't been mad. but i felt like i lost control and my mind...i felt like i wanted to hurt him...but when i think about it...i think it is unfair....he didn't do anything to deserve this treatment. well after i told him about the past. he was disappointed. we didn't become boyfriend and girlfriend until after so it's not like a cheated, we were still getting to know each other. what can i do now? he was bumbed and said that things would probably be different had he known then, but that all that matters is us now. we are very close and have a great marriage, but i feel like these events have damaged a part of us. i wish i never would have lost my cool. i wrote him a letter since he will be away for a few days. he hasn't read it. but today we both made attempts to move on with our lives and focus on us. i feel embarrased and guilty, because i could have prevented this from escalating. was it too late to be honest? should i have retracted and denied this? i had been wanting to tell him, but i felt i couldn't bring this up. the way i brought it up was not the way i had wanted to bring it up...i was angry and hurt. what should i do? has this happened to anyone...where they are dating multiple people...well i only went out with him once...before my husband and i got in a monogomous relationship. will my husband look at me differently now? i love him very much and wish i hadn't hurt him so much with this information. thank you for your time.