I feel dirty and ashamed?

hello this is the first time im going to talk about this this happened to me when i was quite young still i was 13 years old and going out with my boyfriend for 6 months plus he kept pressuring me to have sex (he was 15) eventually he started saying if i didnt have sex with him he would keep slapping me (not hard but he would slap my cheek when he said this) anyway he assured me he loved me and all that stuff so eventually i thought well we have been together 6 months plus now so im not a tramp for sleeping with him and he isnt going to leave me because he stuck around right?

So I end up doing it with him, in a very dark room i was scared and apprehensive and had no reason to doubt him he kept getting up during the act i assumed to do something with the condom etc or to just adjust himself i didnt think anything of it because i had no experience to compare it to :( but it turns out he got up to sneak his cousin into the room ( i had my eyes shut the whole time frankly i didnt enjoy any of it and found it quite upsetting, but i thought he loved me and i needed to do this for him) anyway turns out they were taking turns with me and I didnt even realise, I mean how is that even possible that i wouldnt know :( anyway when it was all over he made me have a shower with him then said he was tired and i told me he would see me tomorrow, as i was walking home two other guys that he had also offered 'a go at me to" told me what had actually went down, they looked at me quite weirdly and said they said no because they respect me and wouldnt do that to me (these guys were younger 12- 13 year olds) but they looked at me like how could you not know! needless to say i was mortified and extremely disgusted in myself, the next day i saw my then boyfriend downtown with his cousin and he wouldnt talk to me they just laughed at me in front of everyone and they told people because other people were laughing at me as well calling me a s*** etc, I moved town not long after this happened (i was in foster care).

My problem is im now 30+ i still dwell on this incident and feel somewhat dirty and unclean about it but i feel i have no right to feel this way i gave my boyfriend permission and how the f*ck could i not know the other guy was inside me how could i not feel the difference :( Im so ashamed about this is this my fault this happened how do i get past it?