If my fiancee seen my message but doesnt reply, what does that mean?

[John Z Wetmore]

She might have multiple windows open on her computer and she hasn't seen your message yet.

Or she doesn't realize that you get panicky when she doesn't respond immediately.

I detect a sense of insecurity on your part that may be a problem for your relationship. If there are issues that go deeper than a slow response to a message, you should have a serious conversation about them before you schedule the wedding date. And talk in person, not over the Internet.