a lie of mine keeps getting brought up. please read.

when me (18)and my boyfriend of 3 years broke up for a month, i sent nudes to a random guy from a different country because i was craving attention and i needed a self confidence boost. (it did not help me with that). it was one boob pic and then he sent me some and i blocked him because i felt uncomfortable and regretted doing it, it made me feel worse and i ended up crying to sleep that night. me and my bf got back together again soon after that, i didn’t think we were ever going to when i did that if i’m being honest. and a couple months in our continued relationship, everything seemed perfect and we were doing so well with our past issues and got to a good place. and then he said this. “you didn’t send any nudes when we broke up did you?” my body went numb and i didn’t know what to do. i didn’t want to hurt him and the relationship, it was obvious it would based on how he is and especially how he phrased the question. i decided since we were broken up it didn’t matter anyways and i was panicking so i lied to protect him and me, and i said no. he was so relieved and gave me a kiss. i thought it would be over but i felt so guilty and he asked me again a couple months later because he forgot he already asked me? and i told him no again because i already lied and at this point if i told him the truth it would make things so much worse. tonight, ( about 7 months back together) he asked again. at this point i thought he knew and i was about to confess. instead i responded “you’ve already asked me this a couple times before.” and he looked confused and said he didn’t. he said again, “you didn’t though right?” and i again, said no. i feel really guilty and i don’t know if he knows, i’m not sure how he would as it was over snapchat and he was in another country so it would be hard for him to even find out. i don’t know what to do. i love him with all my heart and i wish i didn’t lie but i know if i tell him the truth now our relationship would never get back to how it is now, which is the best we’ve ever been. i feel like telling him the truth would only provide me with relief and cause him and our relationship hurt.