Am I overreacting about my fiance asking me for large sums of money?

My fiance and I have been together for 5 years and have admittedly always had this "my money is your money" way of looking at our finances. We have separate finances but if he needs money, Im right there. If I need money, he's right there. That sort of thing. But lately his spending has been a bit worrisome..

I have kids and work FT. Therefore, I got cut a good check for taxes. He worked FT but barely brought home $1000 for taxes. All of his money is gone already. He spent it on "fixing" his truck, despite the thing not being fixed at all after having gone to the shop 3 times. It's just a money hog at this point because he keeps getting different diagnostics and the issue is never fucking fixed. He's been working on it for over 5 months and it's not even a nice truck or worth the money he's putting in to it. But anyways, all his money is gone within a 3 day period.

Prior to this we had discussed opening our own cleaning service business. I have business owner experience, he does not. I also have cleaning experience, he does not. Well he did "research" and decided we needed such and such things when we didn't at all but he had to have this shit and next thing I know, I'm $3k down because of all the stuff he bought for the business that we did not need at all. And now he's asking me for $1100 because he found a vehicle that he wants. It's $1100 after trading his truck in for it. Panels are completely rusted out in the back quarters of the vehicle and supposedly only needs a new battery but I'm not buying that at all. I'm willing to bet there's a lot more wrong with it based off the overall appearance and mileage. And I guess I'm just becoming really frustrated and it's starting to strike me as a red flag, despite him never giving me the impression he's using me for my money before this point. I guess I just don't know what to do. I did tell him I'm not getting him that vehicle and he keeps trying to get me to say yes..

Am I overreacting about my fiance asking me for large sums of money?
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