wife and girl bestfriend

I was not aware that my husband had a girl bestfriend. They've been co-workers for more than 3 years. He doesn't have plans on telling me about her until I found out. Why would he keep it secret if it is nothing romatic?

He even lied to me about her. Told me he's gay-turned out it's a SHE! They called each other "BOSS". I talked to her on the phone but she didn't answer whe I asked her what was going on between them. I immediately confronted my husband and he told me I was being paranoid. At this point, I know he would deny it. I found out later on that he apologized to the girl for what I did. I'm not sure if she deserve it.

I investigated further-installing spy apps on his phone and checking his facebook account.
The next day, the app forwarded text messages from the girl - such as, " where are you?", "You're late again. You should know your priorities." , "you should have went here (office) straight ahead instead of bringing her to work."
I was pregnant with our second child during that time and I was so emotional. So I called him right then and there asking him what the text messages mean. I even texted the girl to provoke her to finally say something.
And when she replied, she said my suspicions were right. They were together for 3 years now. That my husband told her we've separated already. She even told me that he doesn't want me. She asked me to stop bugging her 'cause I lookmoserable. I was so shocked i don't know what to do.

When my husband arrived, he told me that the girl showed her the conversation we had. He said he cursed her for telling me those lies, that she just said it to get back at me for saying rude things to her. Because of that, the friendship was over. He said since that incident, they have become casual to each other.

But I just can't move on-seeing pictures of them so close together. I started to feel insecure because that's not how he holds me in public. I checked on his search logs on facebook and he was spying on her ever since they met. The weird thing is they are not friends on facebook and her profile is in private. So there's no use spying when he can't see anything.

There's this one time when we saw one of his colleagues and he asked my husband why he isn't with the girl. Can you imagine?

He apologized for not telling me the whole thing and assured me that whatever it is that he lied about, he wasn't lying when he said they didn't have an affair.

After all the fights we had and all the explanations, I gave him another chance, for our kids. I thought I will not feel insecure and so emotional once I give birth but just the same, I still don't know what to do, how to feel. Am I overthnking? Am I just being paranoid?
Why do the memories of the pictures and all these things never go away?
I wanted to appreciate everything he does to bring back our relationship to normal. But at times I get too sensitive when we fool around when it didn't use to be an issue.
Do you think my husband really had a thing on her? Does it matter a lot or it's no big deal. I still want to know he truth even if it hurts. Please please tell me...