The speaker begins with a small, odd confession: they tried using a stone as a boat, and it wasn't much good in the water. A stone sinks, of course, so the line reads like a admission of a doomed idea, something heavy trusted to do a job it was never built for. It's easy to read this as a image of how the speaker has handled their own feelings.
The next lines turn guarded. "No open mind shall keep / my best troubles" suggests the speaker won't hand their deepest worries over to anyone's curiosity or sympathy. Then comes the strangest move: sending for a plasterer "to counterfeit a heart." Plaster is a surface, a smooth cover for something underneath. The speaker seems to be building a false front, a heart that looks right while the real one stays hidden.
The mood is dry, wry and a little sad. The tone is calm, almost joking, but it hints at someone holding a lot back.