Try to think of Constitutional Law as a group of principles, not a bunch of rules. Try to remember what principle a case exemplifies. Do not worry about memorising quotations. Sit down and read the opening chapters of Wade, or de Smith. Bagehot is super short and still relevant, so read him in bed.
Your grades are pretty good, and you are only in your first year, so try not to worry too much.
I think that law firms may vary in how much attention they pay to results in modules, but by and large if you get a 2.1 or a first, that makes you a competitive candidate, assuming you press all the other usual CV buttons.
In my chambers we don't care about individual module results, we look at the overall picture.
Good luck!