Boyfriend was cheating since the start

Please pardon my language but, my boyfriend of two months has been cheating on me since the start of our relationship. We are both 16, and I'm his first girlfriend. He says he loves me dearly but I don't know anymore.

I've had a gut feeling that something was up about him and only this one female friend he had who's 24 (yes 24)with a boyfriend. I always used to ask for his phone to just check and reassure myself that nothing was going on between them. He always tried to hide his phone when I asked or threw it across the room. Whenever I did get ahold of the phone though, he would always turn it off or just grab it from me.

I thought this was suspicious but then he told me not to check certain apps or certain group chats. I really did get suspicious, which his friends were telling me that I was just jealous or that I'm controlling. So I just stopped asking for it because I was thinking hey, maybe they are just friends and I'm a little controlling (I had just came out of a relationship where my boyfriend was cheating on me for a month with five other girls).

Well, I got his phone yesterday since he finally agreed to let me look through it. I open his kik and he says I cannot look through the girls messages, his group chats, or anything really- while giving me five minutes to look through the messages. I open the chat with his 24 year old female friend and he tries to take his phone away immediately. I manage to keep ahold of it and I saw that she was calling him hun, talking dirty to him, saying how his lips were so soft, and just trying to do all these things. He even told her how "he only trust her and that she was special". I was honestly about to cry.

His friend was with us, and defended him by saying "They were just joking". By this point, I started walking away from them but the friend called me to come back and my boyfriend didn't try to get my attention or get me to stay, or say it was all a big misunderstanding.

I asked the friend to leave us alone for a bit, so we could talk things through. I asked him if he was cheating on me and he confessed. He had been cheating on me with this girl since the start of our relationship.

He walked me home after that. He didn't hug me, say "I'm sorry", or honestly anything. I went to my room take everything in and figure it all out. We started arguing about the whole situation. He messaged me a few hours later saying he was sorry and how he loves me and that he is scared to lose me. He told me that he would stay faithful, always comfort me when I'm upset, things along those lines.

I messaged the woman and she said she had a huge crush on him, that he was her best friend and such. I asked my boyfriend to block her, which he says he did. I asked the woman if it was true, but she said "I wouldn't let him leave me, he's my best friend!" And she sent him this long message about how she loves him and that she understands that he wants me over her.

My boyfriend is trying to convince me that he loves me and that he's genuinely sorry for what he's done to me.

By this point, I'm just so done. I wanted to stop this whole thing and just say "I'm sorry but you broke my heart. It's over".

Four hours later, the woman messages me again saying how I shouldn't leave him and that he always talked about me and that I'm very special to him, and that he loves me to death.

This just got me confused. I don't know if I should break up with him, because I don't want to shatter his heart.

Please help me, ladies of glow. I know I should leave him but I don't want to hurt him. I don't know if I should trust him.