When he says "My will to her consent is but a part; An she agree, within her scope of choice Lies my consent and fair according voice." He is saying here that his consent to Juliet and Paris' marriage is "but a part" of the decision, so he believes that her choice is also needed. He says that, while his "consent and fair according voice" may play a part in her choice, she still has a choice, free will.
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There is no way to say rather or not someone will regret a life choice, later on, even if don't regret it after making the decision. But one way is before doing anything, is to think it through. For how every long it takes. Ask yourself, "Is this really best for me?", "How will this choice affect others around me?", "Is this really helpful in the long run"?. Asking questions like these, may just help come to a decision. If you decided this is certain choice is the BEST for you. Then you should go for it. While you can't be promised you won't come to regret the decision, it you feel good about it and have thought about long-term consequences you should do what feels best.
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some creepy and spooky onomatopoeias: BOOOOO GRRRRRRRR WOOOOOO AWOOOOOO etc.