Sandman is on the right track, with his observation.

This guy in your office -- the 23 year old -- was inappropriate with you; the way he initiated flirting with you at the office while still with his girlfriend.

I think he got ED b/c he was not ready to take his connection with you to that intimate level. It was nothing you did. It is all about his dishonesty with you from the beginning.

I feel like, he used you from the start. He is very immature, to flirt with a single, available woman like you, while he is already in a relationship with another woman, and lied to you about it until she probably dumped him (that's just a guess).

The way he acted interested/not interested (aka "hot and cold") with you, is also another red flag. A sign that he is not emotionally available to you in the way that you deserve from a man.

I am sorry that you had this experience. I have had this experience with men multiple times. Unfortunately, there are a lot of immature men out in the world.

Please do not blame yourself for any of this. You were single, available, and ready to be in a real relationship. This young man took advantage of you and hurt your feelings because he is immature.

I understand the panic attacks you have, are a reaction to the stress and anxiety from everything he put you through. I also experience panic attacks like the ones you describe, after I experience stress from a relationship that's gone bad. Some people are just wired to respond that way to stress. It's ok.

I don't think you did anything wrong. He wasn't the right partner for you. He took advantage of your good nature and your attraction to him, because he is immature and selfish. Not all men are that way. Don't give up hope. I think you deserve to be with a more mature man. Don't let this 23 year old man control how you feel about yourself.