You can add a little gum tex or tylose to your fondant to make it stronger. Shape your initial and let it dry for a few days depending how thick it is and it will stand up.
however you need to anchor the initial to something.. I usually use floral wire in a heavier gauge, that can be found at Michael's. insert the wire into a straight part of the initial with some sticking out which will be inserted into the cake once the fondant is dry.
you can use a coffee stirrer in the cake to hold the wire.
Mayaverga, make two just in case one breaks. It's better to be prepared.
You can also consider doing it with Royal icing...just tape your monogram design on a piece of cardboard, tape wax paper over, then pipe the outline, thin the icing, and fill. Let it dry, and voilà! 
No matter what method you use, definitely make an extra (or two).
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