guilt/ unsure how to feel

this might be a little long so i understand if you skip haha

my boyfriend and i have been together for 3 years and it’s amazing. when we first met (before we started dating) i was talking romantically with him and my ex at the same time which i cut off before we started dating, it resurfaced about a year into our relationship and i lied and said i didn’t talk to my ex at the time because i didn’t want something so small to me that was before our relationship to hurt our built and happy relationship, i eventually came clean and told the truth, which we worked through and it’s in the past. however about a month ago my boyfriend and i were having a hard spot and arguing so i opened up to a friend (male) who also knows my boyfriend to vent which led to me talking about my ex (who he also knows) who i grew up with. i talked about old times together and just the relationship as a whole (how it was, why we broke up, etc) i said that it’s weird to think ill never speak to him again or see him after i graduate (i graduated in may) and the friend kept telling me there would be nothing wrong with re opening contact and talking (which i never did). the friend also was telling me how the ex missed me and would talk about me frequently which i replied “aw that’s sweet” which i didn’t see anything wrong with. later on at a senior beach trip the ex was there and texted me asking where to park which i blocked him and told my boyfriend. we have mutual friends so he was around me and my friends all day. we were never alone or talked alone but we did have brief small talk in a group. after the day was done we went our separate ways and i smiled bye. after that my ex had told my friend that i opened up to that he missed me after seeing me again. the next day the ex was actually hanging out with the friend i opened up to and snapped me off of his phone and was flirting and said that he will always think about me, i replied that we broke up for a reason and we are both in happy relationships now and later told the friend i didn’t want to speak about the ex anymore and that at the end of the day my boyfriend is who is for me and i would never step out of my relationship. a couple weeks passed and the friend had texted me and tried to make it seem like my boyfriend was cheating (trying to set us up and make us break up) when my boyfriend confronted him about it the friend told him that i had said i missed my ex and that i would always have an special place in my heart for him which i told him was not true. it passed over however i now feel extremely guilty because he told me if i did say it and if i ever gave me something to worry about with my ex again, he would break up with me. i now feel extremely guilty because i didn’t tell my boyfriend that i had opened up about the ex to my friend (or ex friend now) or that my ex snapped me off of his phone. i never texted the ex back and i never stepped out of my relationship but ive had extreme guilt and stress for about a week. my closest friends say that it’s nothing to worry about and i didn’t do anything wrong, so why do i feel this way?