心細い literally means “the mind is thin”. Many dictionaries describe this い-adjective as “lonely” or “loneliness” - however 心細い doesn’t really mean “lonely” in the way we think of it in English - a better translation might be “to feel weak or vulnerable.”
心細い is mostly used when we feel anxious because we do not have or know anyone who we can rely or depend on. For example, if you move to a new area, you probably feel vulnerable since you don’t know anyone who can support you yet. And this feeling of support can be related to things, like money. If you are old and don’t have much savings, you’ll probably feel anxious about your future. In this case also we can use 心細い.
The antonym of 心細い is 心強い, or “to feel secure”, which literally translates to “the mind is strong.” You might think it would be 心太い since 太い (“thick, chubby”) is the opposite of “thin” - but don’t be fooled!
Anyone would feel lonely if they lived in a foreign country where they don’t know anyone.
I feel anxious because my savings is less than 100,000 yen.