Congratulations on your offer! It is not uncommon for attorneys to feel this way once they receive an offer, notwithstanding how excited they are about the new opportunity. This is especially so because offers always pit the familiar against the unknown. No matter how interested you might be in a prospective opportunity, it can be difficult to leave your comfort zone and test out new waters.
My advice is to really go back and reflect on the considerations and factors that led you to begin interviewing for a new position in the first place. This will allow you to assess how closely this opportunity comes to matching the position you were ideally seeking when you began your search. For example, is the practice group as busy as you would like it to be and is the work at the caliber you desire; do the attorneys seem to work well together; is this a group of people you could enjoy working closely with; do you believe there will be genuine opportunities to distinguish yourself from the ranks for promotion purposes; does the firm have a strong record of associate retention; etc.
These are just suggested considerations, as the list of factors differs from candidate to candidate. Some people care deeply about salary considerations, while others are more focused on substantive work experience, client contact, workforce diversity or other considerations. However, if you take the time to go back and really reflect on what led you to begin your job search in the first instance, and can then honestly assess how closely the new opportunity matches your initial interests, I am confident that – one way or another – you will have a better sense of how to respond to your offer.