In Shakespeare's Sonnet 30 there is a tone of regret as the speaker thinks about his past personal losses and sorrows. In lines one and two the speaker provides us with a condition (whenever he silently thinks about memories of the past). Lines three and four then provide a sort of introduction for the next eight lines that come after the colon at the end of line four.
Here is a breakdown of the wording. The words "I sigh the lack" mean that he "sighs about not having", and "many a thing I sought" means "the several things I used to try to find". Thus he is saying "I sigh about not having all the things I used to try to find". In line four the words "And with old woes new wail" mean that "And old woes cause today's crying". What is he crying about? According to line four, it is "my dear time's waste", which means "the time he now realizes that he has taken for granted".
In the next eight lines of the poem after he will then go on to list several of the sad things that he has hinted at in lines three and four. They are the things which cause him to feel a new wave of grief and regret whenever he sits in sessions of silent thought.