I made white chocolate ganache a few days ago and it never hardened like it usually does. It behaved a bit more like buttercream so that's how I treated it. I was able to get my sharp edges and use the viva method to smooth it out. However, when I covered it in fondant, it ended up bulging I guess from the weight of the fondant. I've NEVER had bulging before whether I was using ganache or buttercream! Luckily the design of the cake ended up covering the bulge. 

I'm guessing the problem was that I measured wrong and added too much cream. Any thoughts? Do you think it might've been something else?

Any ideas on how I might've dealt with it better would be appreciated. Should/could I have added something to my ganache to fix it? I didn't realize it wasn't hardening until I put it on the cake already. It looked perfect until then. It also tasted fine.

TIA

If the ganache won't set, try chilling some of it.  Then mix that back into the room-temperature bulk.

If this doesn't work, then you need to add more melted chocolate. Hard to say how much because there could be various ways to add too much cream in the first place.  You should always have 25% more chocolate on hand for such emergencies anyway.