You are being cheated on. Why would you forget about that, even for a minute?

I would never allow my husband or boyfriend to develop or maintain an emotional relationship with another woman. If you are going to permit that, why would he ever stop? He is grooming you to be the nice little wifey that sits at home while he does whatever he wants. That kind of thing does not happen out of the blue. He's testing limits to see what he can get away with. You must have an awfully passive personality to even be asking this question.

He's teaching you what kind of person he is. Why aren't you getting the message?

My best advice is this: Pack your shit and hit the road. Block him from calling, texting, tweeting and anything else you can think of. Ignore him until he goes away, because if you take him back then you've only taught him to be sneaky. He's already shown you that he can't be trusted, believe him now instead dragging this out, particularly when you end up with kids to worry about.

If he's hiding it from you that's not a good sign. A part of the reason he's being secretive about his friendship with this other may be because of your jealousy. He may be afraid of your reaction if he communicates to you about his friendship with her. Another part could be that he is actually interested and/or has some feelings toward this female but may have feelings for you too and so by telling you (fully) about their friendship he may risk losing you. It's wrong. Absolutely. Sadly some people operate under this thinking. Another part may be that he is cheating on you. Also sad but again it could be. If I were you, I'd have an honest conversation with him. Remember though, if he tells the truth, try to not show your jealousy even though you may feel it strongly. When talking to him try to not be explosive or accusatory. Simply tell him how his friendship with her makes you feel and make sure you define what is appropriate in terms a friendship with another female and whats not. You would be surprised to know that some people have different boundaries when it comes to friends of the opposite sex. One time, I was surprised to notice an older chick was getting friendlier and friendly with my beau through out the afternoon even though it was crystal clear that he was taken by me and we were expecting a child together. At first she started with compliments in front of me. After some time my intuition started to alert me about her but interestingly not about my beau. After we left, my guy told me "I was surprised you didn't do anything when she started rubbing my back" You can only imagine my response from there lol. I had no idea the woman was doing that. First her husband was there. Second my guy was taken by me and I was clearly pregnant with his child. Later I learned that she has an "open" kind of relationship. Not my thing but clearly it was hers. My guy on the other hand is aware of whats okay and whats not. Why? Because I have firmly and respectfully communicated that with him so if anything ever goes down he knows I don't play as much as I care for him I will leave because infidelity is ZERO tolerance for me. I share this with you in hopes that it will help you to see :)
and as scary as it may be to potentially lose this guy, remember there are plenty of fish in the sea :). Breaking up is awful and you may have to endure some uncomfortable emotions but afterward you gain strength that you probably never knew you had not to mention you become wiser in selecting a mate in the future. Plus if he is being unfaithful wouldn't you rather know now than five years and two kids down the road? Just saying...

So my advice to you is to NOT forget about your feelings. You are important. You are worthy. You are special. You deserve to have the same love reciprocated back to you. If he can not provide that to you than you guys are not meant to be. If you believe he is worth it, talk to him. Be honest with him. Ask him how he feels about your relationship with him. Let him know that youre making a commitment to work on your jealousy but that you also are not comfortable with secretive friendships, messages, etc.