I personally deal with the guilt (though quite minor compared to what you seem to be experiencing) by being very generous with my bought resources, but ultimately if it doesn't make you feel good to buy your pets, you don't have to. the only purpose to buying items is to increase your enjoyment of the game, so if it's not doing that for you, it's totally within your right to abstain from it
Being generous with my bought resources does tend to alleviate most of my guilt. I ended up trading the UC, not because of guilt but because I wasn't really connecting with it or the name when trying to create a character for it, so hopefully it makes the person I traded with happy (or help reach their trading goals). I feel fine now, but when I posted this, I was sort of wracked with guilt, ngl. Not so much anymore. I'll probably keep buying NP once in awhile, but maybe not UCs again. Part of me wishes TNT would make it allowable to buy NP, but then I remember what happened to the Gaiaonline economy when that happened with their currency.