If I buy my dream guitar my fiancee is threatening to leave me over it.
Why is your fiancee threatening to leave you for wanting to buy your dream guitar?
She said it's stupid and not practical and it costs way too much money.
I know it's your dream guitar but it sounds like it's very expensive. A large purchase like this really should be discussed with your fiancee because it will cost a lot of money. Large purchases should always be agreed on by both partners.
Maybe, but I want if now.
If you have money problems you should set up a small savings account for that purchase. You really need to make sure you both are on the same page with expenses so you don't have a bad dynamic.