What do y'all think I should do?

Almost 1yr ago I cosigned on a truck with my SO, the legalities never matter much so I never looked into any of it. Well fast forward my SO joined the military and hasn't even finished basic. We agreed while he was gone that I would take care of his truck; driving it, oil change etc. Last weekend I was helping my parents move some stuff and my SO's brother told their mom; why idk. Anyway she calls me up yelling at me that, the truck is not mine and ended up threatening me and saying she would make sure I pay for any damage that happened to the truck and that I was no longer allowed to drive. 
I wasn't in the mood to handle all of it so I didn't really respond. Later after doing some research I found out that I'm legally registered as a co-owner on the truck and that I have to be on the insurance BC of that. In other words my SO and I have the same rights to this truck and his mom has no right to tell me what I can or cannot do in reference to this truck. 
I have to see her tomorrow and plan on talking to her but since she is also my landlady I'm afraid she won't be happy and decide to tell me I can move out. 
Also she is making sure all of my SO's bills are paid, so I know she is looking out for her son but I'm really debating on not telling her yet BC I don't want the next phone conversation with my SO to be him getting irritated and upset BC his mom can't handle something and I don't want to cause any drama but I'm afraid it will.

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