Does the sentence 'There's only so much I can do' mean "I've done everything I could" or "There's still something I could do but my power is limited and it may not be enough"? In other words, is that "so much" already done or not?
There is only so much I can do
does not formally address the past at all. In context, it may be possible to infer that nothing, something, or everything was done in the past, but the words above on their own are silent on the issue.
Now that I have two children to care for, there is only so much I can do
certainly implies that I used to be able to do more. In a formal sense, it only speaks to a difference in past and present capacity and is silent on action. However, it usually would be intended to imply that I previously used to do something without necessarily implying that I used to do everything.
There was only so much I could have done
does not imply anything about the present or future. Again, it does not formally say anything about action; it formally speaks only to past capacity. Usually, however, it would be intended to imply that something, but definitely not everything, was done in the past.
They are vague usages that formally speak only to capacity, but usually imply partial action.