It is true that you don't have to give all details about everything, when in a relationship. Because you shouldn't just depend on the one you are dating. BUT, there are Things that you should tell the person you are dating. Meeting Your ex who is also your first love multiple times, and also spending a long time with them, is something you SHOULD tell them about. Keeping it a secret, and have Your bf or gf find out about it through other means, not good. This is how I see it, my pov if you will. It seems very logical atleast. Then again, I have never been in a romantic relationship though, so I don't know how it truly Works.