I don't know who told you that stuff about subjects in first position mean something special but consider
Rule #2 of German word order:
- German grammar in general doesn't care about the position of items in declarative clauses.
Of course, you remember Rule #1 of German word order:
- The core of the predicate verb is always the second item in a declarative clause.
So, what's this word order thing about then?
The order of items in a clause is used for emphasis. The first item has most emphasis (it's the topic) and the last one has second most emphasis. For your example, there are six possible permutations:
Du fehlst mir so sehr.
Du fehlst so sehr mir. (not wrong but sounds odd)
Mir fehlst du so sehr.
Mir fehlst so sehr du. (not wrong but sounds odd)
So sehr fehlst du mir.
So sehr fehlst mir nur du. (
mir du sounds odd, hence the
Those sentences all mean the same thing, with different emphasis.
You are missed by me so much.
A good German-English dictionary will in fact translate fehlen as to be missing, and not as to miss. The latter is vermissen in German.
Du vermisst mich so sehr.
You miss me so much.
As you can see from that, fehlen is a bit special. The "action" of fehlen is "done" by the missing person or thing (the subject), but in reality, the person/thing does nothing. It's just not present. That whole action is about a perception of "der Fehler" (deficit) someone receives. And that's why this person is the dative object rather than the subject.