I’ve been commuting by bike for 20 years. I would say that 15 miles each way is eminently doable for most people.
Lots of good advice above. And I shall reiterate the pertinent bits from my point of view.
Gears:
This will definitely help in the long run. 15 miles with some climbing is, as you’ve found, OK on it’s own. But if you can sometimes sit and spin in a low gear and take it easy, you will find that you don’t get to the end of every week feeling crushed. I incorporate most of my training into my commute, and sometimes that means dawdling into the office at recovery pace. It adds surprisingly little time to the journey.
Luggage:
Depending on what facilities you have at work, and what you’re required to wear, I would get a large saddle bag or rack-top bag to carry your stuff in. Day to day I use a large Ortlieb saddle bag to carry underwear, polo shirt, wallet, and sarnies. Trousers/shorts, shoes, etc all stay in my locker/kit bag at the office and get brought home every few weeks for cleaning.
Bike setup:
At least 28mm tyres, preferably 32-25, with full (looooong) mudguards. While I don’t have them fitted all year, they make a huge difference compared to little crappy clip-on things. A front guard with a deep flap will keep spray off your legs and feet. This makes a huuuuuge difference to how effective rainwear/overshoes are at keeping you comfy.