How do I decode scribbled out writing in pen?

You could try shading with a pencil edge, over another piece of paper. Whatever was written, may have been deeper, and stand out…

Was the original writing in pencil and the scribbling in pen? Was it like a felt-tip pen or a Sharpie-type marker? Was it actually scribbled (many line markings back and forth) or just blacked out (like with one big wide stroke)?

Was it struck out like it was censored by security, where you can see the rest of the message apart from the blackouts, or was it like your fiance crossed out his own message?

I’d say your chances are better if the original writing and the crossouts were done using different writing tools than if they were the same, such as pencil on pencil or ballpoint on ballpoint. But I’m not a detective and am just making guesses here.

Welcome to Fluther.

You could try – carefully! and on only a small part of the writing (such as the signature, that you don’t need to “read”), and after that the scribbling – to see if any of it is water-soluble. If you’re lucky then the scribble might be water-soluble and the underwriting more permanent. (It’s unlikely, especially if it was all written with the same instrument, but it’s out there as an idea “just in case”.)

I would hold the card up to a window on a sunny day (thus creating a light box). If the original message is in one type of ink/pencil and the subsequent scribbling is in a different type of ink/pencil, you may be able to see the difference between the two and read the original writing.

My wife used to work in a prison and all outgoing and incoming mail was inspected first and redacted if necessary. Was this the case here?

Turn the paper over. Perhaps the original words were written on a softer material than the scribbling, and the words should stand out.