Lots of great suggestions in the previous posts. Merino baselayers. Sometimes it helps to have different sizes to layer them(thin). If my core is warm/dry/generally comfortable, I find that I can get away with miscalculations in the outer extremities. Since gloves can be a tricky choice on bicycles, focus more on your wrists. If your wrists are warm, your hands are more likely to be happy. I use either a merino glove, or occasionally some waxed Kinco "drivers", which are kind of a non-insulated ranch-style glove for wet/cold big days. Merino socks, but thin enough to leave space in your shoe. Tape over any ventilation holes in shoe sole. Neoprene shoe covers are also appropriate in many climates. Since you have a backpack, carry a merino balaclava and a plastic shopping bag to shove under the front of your jersey. These 2 backpack items will maybe go unused, but can both save your day if you miscalculated somewhere else. I've also been rolling with an extra set of gloves for myself, or even to share. <--gross, but always welcomed when needed!
Some other thoughts:
If you insist on wearing shorts instead of proper winter tights, apply large amounts of Aquaphor to your legs. It repels water, wind, and conserves some body heat.
Showering. I shower before every ride. When showering before cold rides, I very deliberately allow time to completely dry-off and cool back down before kitting up and heading out.
Stretch. If you get cold, stop, and stretch some more.
Avoid tactics like double-socks or double-gloves. Blood circulation and moisture management are more important than R-factor.
Oh man! I almost forgot the BIGGEST cold weather riding tip. CALORIES. Eat MORE before, during?, and definitely after your adventure. Fat, protein, and slower carbs.
Slow winter riding with proper recovery pays huge dividends when spring fever hits.