Nervous over my larger boat purchase. Is this normal?

Not the only one and not the only one to overestimate the difficulty. A well behaved larger boat often maneuvers better than a smaller one. The weight actually makes it easier in many cases and less moved by current and wind. Any two boats are different, but I wouldn't think of the 45 as more difficult than your 36.

Had the same feeling when I was very much used to my 33’ sailboat, then took a job operating a private Johnson 70.
Got a quickie checkout by another Captain, then solo..
Got used to it somehow, but not coming back to the slip at Bahia Mar, getting the boat in position, then run down to the aft deck from the fly bridge, activate the controls and backing her in without seeing much forward or the other side. It had 25hp bow and stern thrusters, easy enough to maneuver, but I always had a Mate, or the owner was helping.
Prefer boats I can operate safely solo, but they don’t pay as much.

I remember when one client I taught to run his single I/O called for another lesson when he bought his first twin. He was all nervous. I laughed as I told him his life just got a whole lot easier. Same thing as you get bigger. It's less affected by wind and current. Add to that the bow and stern thrusters and I say Sleep well tonight. You're going to enjoy your new boat. Congratulations and welcome to YF.

When I see these jockeys who slam their boats into their slips it looks cool. To me it just tells me that's someone who never had a gear linkage come lose or break. I always tried to impress on people I taught that Slow is Cheap.

Once an item is moving you're correct, but it takes more effort and time to get things moving the heavier they. A cruise ship is a lot less affected by a 5 kt. current that a Boston Whaler.
P.S. What's the difference between an air current and a water current? Resistance.

According to a book I read on boat handling, by some US Navy Admiral, the current is 5 times stronger than the wind.
(5 knots current is the same as 25 knots of winds)
Great book by the way, highly recommended, how to maneuver subs in narrow channels and other goodies.

Again resistance. In that case by the density of the medium.

And a rigid rule that no one does anything, no guest tries to help, unless instructed to by the captain. In fact, our rule is that they stay seated in place unless asked. Most of those on board in our case, know what they're doing and know how to help but can't have everyone moving around and getting involved.

I find a good skipper's command when maneuvering/docking/anchoring and so forth is "stand by".
I use this whenever a spare hand on board or on the dock asks if assistance is needed.
Sounds better than "no", and who knows, I may need assistance.
So "stand by".

This will avoid when a well minded hand pulls the bow in just when you wanted to keep your stern where is was.

You should be fine going from a Monk 36 to 45 Symbol.

Only go as fast as you are willing to hit something

Neutral is my friend

I find "stand by' a bit ambiguous and nautical unless dealing with a deck or dock hand I intend to use shortly, and I'd keep one eye on them after that. I prefer to be more direct, which once came to "Drop that f------g line" when a dock hand wanted to debate if he should drop a line that was keeping me from getting out of a bad situation. 'Take this' 'Do that' as short and plain as it can be work better. I'll tender apologies after the boat is on the dock if they're offended. Right now is when directives need to be followed on a boat. My wife's cousin was sailing with her husband through a gale once and he told her to release a sail Her response was 'let me get the coffee first' as she headed below.

There is more to it than Stand By to be sure.
While I know it is not always the case, I find many folks at marinas are a bit nautical.

I DK about that. I remember my boss on one boat was always intrigued by my words and expressions as he tried to figure them out and learn nautical speak. One day I said "Check out the Russian sub". After a few minutes he said "Ok I give up. What's a Russian sub". to which I replied "That boat over there with the red star on the conning tower". I've known a lot of "boaters" who knew little more than how to mix a martini. I wouldn't want them guessing what I mean.