I was previously in a relationship where my feelings for the person were intense and full of meaning when they weren’t with me. But when I was actually with that person, I felt indifferent to them at best.
If you want to test and see how much someone or something means to you, the true test is when you’re around them. The purpose of being with someone is to be with them, in their physical presence, so trying to work out how much someone means to you by seeing how much you miss them is useless. Since the goal of being with someone is not to be apart from them, the value of something, and how much you want to be with them, is when you are with them. It shows itself when you have it, not when you don’t.
Sidenote: When people say I didn’t appreciate them (or it) when I had them’, there may be a reason for that. Sure, that person may not have got to a point where they have worked out the truly valuable things in their life, but it might also be their subconscious telling them that this thing, or person, is actually not the best thing for them.