The poet is feeling said and sorry for himself because he is in love with a married woman he cannot have and goes into the garden in the Spring for comfort. But he is too lovesick and sad for the Spring garden to cheer him up.
He thinks that the only cure would be to become a part of the garden.
He suggest that lovers like him should take his genuine tears to compare with the tears of their own partners. If they don't match then their partners tears are not genuine.