AITAH for texting my ex girlfriend behind my boyfriends back

I know the title sounds bad but let me explain - my ex girlfriend and I broke up 2 or 3 years ago. We're still in the same friend group. My boyfriend was friends with her. But all in all we agreed to cut her out because she was damaging our relationship.

I will admit, I am in the wrong for this; but I texted her behind his back during a fight between the two of us. This was because - and what I told him - I needed to talk to someone who understands. My ex saw me through the worst times of my life. She witnessed me trying to grow into a better person than I was. Let me be clear, I have absolutely no feelings for her other than thankfulness that she was there.

1Anyway, after the fight we continued to talk on and off for about another month. When my boyfriend found out I did jump to a lie and say we were only talking for a week or two because of drama with my family. I did clear it up though and tell him the truth. I didn't let him read the messages. Not because I have anything to hide, but because he blows up at the tiniest things when it comes to reading my text.

All in all, we fought. He says I've been lying and he can no longer trust me. Fair. I did lie. But I also explained to him that she was my best friend. That she is as emotionally there for me as his best friend. He doesn't want to listen to the fact that she is literally just there because she and I understand each other on a different level. I'm not saying that to make it sound like we're still in love. We aren't. We dated for a long while. She saw my life turn to shit. I saw hers turn to shit as well. She saw me work through my mental issues affecting our relationship. Just as I saw hers. I have no romantic interest there.

I just needed someone there. Especially since it's summer break. I apologized. I told him why I did what I did. I told him it wouldn't happen again. I understand why he's mad. But is the whole "I can't trust you and never will" really necessary? Am I really the asshole? She was both of our friend. He used to confide in her too until his newer best friend. He knows I wouldn't do anything God awful and he knows she would tell him if I tried. Is it really that bad? Am I really that awful?