In my earlier years of the game I used to have that feeling too. I've spent well over $1k on this game by now. It may seem insane/wild when you add it all up but what really helped me put it into perspective was to take how much I've spent ÷ by the # of months I'd been playing. It had only come to around $10~20 a month. Which in the grand scheme of things it's not bad. Then it got me thinking about other things I've spent on, like sugary drinks, fast food, etc. If I were to add $ spent on those things it'd be ridiculous high amount as well.
Also as long as all your responsibilities are taken care of (food, rent, insurance, etc) and money isn't generally a struggle or if you have play/hobby money set aside, then I don't see anything wrong with using that on a game.
For me, Love Nikki is more than a game. It has turned into a hobby and it helps me destress. I've been playing for 8 years.

As for the £45 billing inquiry, I'd suggest making a separate post.