How can we know when we should do hishtadlus and when we must have Bitachon?
As a rule: When something is within your ability and in your possession is an area where you may not rely on Bitachon. For example, someone cannot say, “I have bitachon that the money in my pocket will just come to my hand,” or “The food will just get from my plate to my mouth.” It’s self-understood that this is just plain foolishness.
The clearest area that calls for Bitachon is where what you need is not within your ability to obtain nor is it in your possession. Such as needing a certain amount of money which you have no way to obtain.
If something belongs to you but it’s lost or stolen, or even if you lent it out and the borrower refuses to return it, you can certainley have Bitachon that your item will be returned.
The only grey area is where something belongs to you, and the person who has it is willing to return it but they’re not in a hurry to give it back. It’s possible in this situation one must put in the effort to retrieve the item, however, it’s also possible that one may have Bitachon here as well.