Every time I tell my boyfriend something I'm not comfortable with, he TWISTS it into something more than it should've been. For instance, we got into it at the car shop b/c I asked him what he's going to do with the leftover car paint they gave him. He said, "I still see we have some visible cracks that weren't covered." And knowing him, he would try and fix it himself.
Mind you, he is not an expert. He doesn't know much about cars beside the tutorials he watches on Youtube and he messed up (really badly) the paint job on my car the first time after accidently backing up into one of our family friend's car. Now, he thinks he can do it all and tells me to have FAITH in him each time I question him about it.
I mean it's my car!! I'm the only one who pays for it each month. I rather get it done right the first time around. Am I crazy!? Should I just submit! He questions my faith in him like it's my fault that he doesn't get things right. Really??
He walked off after calling me an Idiot b/c I told him I didn't want him attempting at painting my car. I know he's trying to be helpful and "fix" the damages that he's done to it, but in no way should he try to repair it himself.
I really don't know what more to tell him. He said I basically am calling him "incompetent" when I said I didn't want him fixing my car. I don't think he's incompetent, but as I said I rather get it professionally done right the first time.

He just believes that he can master anything overnight and no one can or should question him about it. Even if he's in no way responsible for it, he thinks in his mind he can do it all.

I need to know what his problem is and what I should do/say to him. I mean I do my best to be firm with him and let him know why I feel uncomfortable. But, he takes it to another level and starts calling me degrading names and tells me I have no trust in him. I know he or anyone is capable of doing it, but it takes some serious practice and commitment. But nope, he doesn't comprehend that at all.
He's all over the place. When something peaks his interests, he gets really OBSESSIVE over it. Spends a significant amount of time researching and buying things b/c of his current obsession. Next day he gets cranky and exhausted from all of it. He DABBLES in many things, but never seem to follow through with it. So, that is why I'm upset, frustrated, concerned, and just don't know what more I can say to get him to understand where I'm coming from...