Thanks for bringing your question to JustAnswer.
It's possible that while you've been telling yourself that the conversations with an ex were completely platonic, it wasn't until you said it out loud that you realized that your bf would be very hurt if he knew.
Now that you realize it, it's time to break off communication with the ex. You know that's flirting with danger. One conversations to catch up on things would have been fine, but keeping the chatting going isn't going to help your current relationship.
So, first make the decision to do the right thing.
Then, try some EFT (Emotional Freedom Technique) tapping to help yourself relax. This is a self-help method that's easy to learn, and will help you quickly feel less anxious.
Here are some YouTube videos that demonstrate it, and you can follow along and calm yourself down:
Here's a basic video that teaches the technique http://www.youtube.com/watch?v=UlCE4P6Ue0w&feature=related (She has a little bit of an accent, but its the best basic video I've found. Just watch this once, then go on to the videos that follow here--
This one is specifically for anxiety
This one is for fear and worry:
It's hard to imagine that such a simple technique can give so much immediate relief, but try it--you will be pleasantly surprised.
This thought is a product of your anxiety, not a fact.
How could that information be used against him? Haven't most people had relationships that ended by betrayal? If your boyfriend has that much control over what you feel you can talk about, it's time to re-evaluate the relationship. You weren't talking out of a desire to hurt him, you were asking how to make sure NOT to hurt him.
Please go try the EFT and lower your anxiety. This thought will fade from importance in your mind once you are calmer.