Hello Romancelanders, please I'd like to have your opinions. I wish I had someone I could talk to offline.

My boyfriend went for NYSC last year and during his service year, we were very cool. The distance didn't affect us, he came home for a reasonable number of times and we communicated everyday. He never failed to tell me how much he loves me and can't wait to be done with service. He had his pop a little while ago and finally came home.
Everything seemed fine and we had a few catch ups but something didn't feel right within me so I asked if he had been with someone else on a regular basis, he denied and I believed him completely. A couple of days later we were both viewing his phone gallery and I noticed a picture in particular, the picture had him and a female friend in it and the setting was a bit too intimate for a platonic one, this sparked some questions in me because he used to tell me about the lady, they met when he went for service and he never gave me any reason think twice about their friendship, he has many friends so I just thought of it as normal. I have never gone through my bf's WhatsApp chats, not that he kept his phone away or anything but it just never occurred to me to do so, I trusted and respected him so much, highest I did was to take pictures and play music. I decided to check his phone after what I saw, I went straight to his conversation with the lady and what I saw left me in total shock, they have been having a sexual relationship, from what I deduced, they started having sex around the third month into his service year. Their chats left nothing to be imagined and they said so many nasty stuff, sent almost nude pictures, they professed love and always talked about being in each other's arms. The Lady in question knows my boyfriend is in a relationship and my name came up in their Convo many times. There was a time my bf came home and we hung out, he actually sent our pictures to the lady and with all audacity she asked if he had sex with me, I can't remember what his reply was. Another time, she told my boyfriend that she has broken up with her bf and that he shouldn't waste time to do same with me, he told her he wasn't going to do anything like that. I was appalled at this point already cos the guy made a complete fool of me, I was like the "small girl somewhere" in their Convo....I told him about all I saw and how he should have done better than that, he was shocked I did and was more shocked that I took it calmly cos I didnt rant, didn't walk out, just had a civil conversation with him. He has been really remorseful and said he is more sorry that I got to see their chats, he said the girl means nothing to him and he told her countless times that they can't be together cos he loves me, he asked if I went thru their chats very well, asked if I noticed he always told her he can't leave his gf.. honestly he did mention that several times to her but he thought he could have his way with her as par nysc fun, it doesn't make any sense to me. He's been really down and said I should take my time to think it through, he said he'd take my decision in good faith cos he has permanently scarred our relationship. I told him I'd need some time alone.
Honestly, I still love him and would love to put the issue behind us but I keep remembering all I read, It might have been relatively easy for me if he did with another girl but the girl in question?? He paraded her as a good and supportive friend while they had stuff going on. I'm really hurt, never been so heartbroken.