Is it normal that someone can change?

i have been with my boyfriend for 3 years now, we are both 22 years old, i met him and his family shortly after we met, his family are wonderful his mom is a lawyer and dad is a real estate, my boyfriend never tried drugs and he is a non-smoker, a year passed by everything was fine, he met few colleagues of his and started to go out on a regular basis with them, and i was a bit worried cause i had never met them, i mean i did but i do not know them that well, so a few months pass, and we had this argument about him going out with those friends that i did not feel so good about, the argument became worse and i kicked him out of the house along with his stuff and locked the doors. he called and called i never answered him... few days passed and one of his colleagues texts me saying " he is with me, we are out of country now, he is in bad shape come take him" i was shocked and i ignored i said well its his problem... few days later that same colleague's GIRL FRIEND that i never saw or heard of texts me a picture of my boyfriend along with this other girl kissing. i was shocked speechless and didn't react. few days passed, he came knocking at my door steps saying sorry bla bla so i told him whats the deal why all this? he had admitted to me that he has started using drugs with those colleagues of his and that it had damaged him and that he was lost now knowing that he never used drugs i asked him why would you start now i mean what the hell he told me he was influenced by them and he fucked up, i told him did you sleep with that girl, he said yes. i was shocked he never cheated on me. plus he KNEW the girl, she was a friend of his.... anyways he told me when you kicked me out i was lost and wanted to forget so went out there took drugs and got drunk and slept with this whore. this is so not him i was surprised. i told him listen we cant be together anymore i am sorry if you have a problem with drugs go to rehab i wont be your rehab so get out fix your self and we can talk when you are sober. he also confessed he was using drugs for the pass 6 months intensively including cocaine and marijuana.

i did not know he was gonna take my words seriously as i thought our relationship was over and he was a drug addict i didnt want anything to do with him and plus he slept with another girl.

few months has passed i didn't hear for him, nor his family, i said to myself well typical i guess. i was hurt and in pain and was suffering to move on.

then, my door knocks, he comes along with his mom explaining to me that he had been to rehab and sober and free of drugs, his mother said he fucked up and did this mistake but he fixed it and he told me i fucked up went out of where i was took drugs but now im sober and i worked hard to do that so i can be with you.

i love him deeply, so i gave him another chance only cause i knew he wasn't this person and i knew for a fact he really went into rehab.

now my question is, do you guys think i did the right decision i mean im writing this a year after this had all happened but i was just cleaning my mac and came across this picture of him kissing the girl and remembered and started crying asking myself but why would he do something like this to me? was it really because of drugs? or is he a cheater who will always cheat on me?

i mean he admitted to everything he has done from taking drugs to that night he slept with this girl to every little detail...

please guys i do not have any one to talk to i just have you here to tell me what i did was right or no?

i mean now after a year has passed he is really focused on his job, we never argue and i do not have any suspicions about him, we are always together, we travel together, we work at the same company and we have the same group of friends. he stopped returning calls or calling those other colleagues of his (he quit his previous job).

but i am just scared...