Nor dare I - An implied prohibition and fear of transgressing it. But jealous also had the meaning of zealous, or attentive, as well as its more traditional meaning, which predominates here. But, like a sad slave, stay and think of nought 11. But also with the older meaning of steadfast, constant, of serious purpose. He dare not think of anything for fear of thinking the worst.
Modern punctuation tends to separate the meaning of the two lines.
Save, where you are, how happy you make those. So true a fool is love, that in your will, 13. Will is often used for sexual desire or performance, or parts. With a pun on the name Will, hence 'In my thoughts'. Though you do anything, he thinks no ill.