This is needless to say really complicated and I feel for you because I've been in similar situations. Some things to think about. Take it for what it's worth.

It's ok to be friends with an ex. You just need to make sure your feelings are mutual as only friends and you need to show your current girlfriend this. Don't lie about talking to your ex or hide any feelings you have, it will only make things worse. I am not much older than you so I realize that having any sort of relationship with an ex isn't easy for people your/my age, but it's really time to grow up at a certain point.

People that are emotionally tied to you always give biased opinions. It's only human. Of course my advice to you right now is biased upon my worldview, but it's less biased, you know? I think you made a good decision by posting here to get some feedback from people who just listen to the facts (from your point of view)

Honesty really is the best policy in relationships of all kind. That doesn't mean you shouldn't reflect and think about timing of saying something, but if you hide a lot of stuff because you worry you will hurt someone you will just hurt them more later when the lies come out.

Good luck.