Layers.
The problem with dressing for cycling is that people tend to overdress. Unless you're cycling below freezing, get a decent windproof shell. You'll be plenty warm if you wear a midweight jersey and maybe a fleece underneath it. That's all you need.
I have a classic wool cycling jacket that's good down to @ -10C. Even that's overkill, at times.
The ting you have to worry about is not so much ambient air temperature as wind. If you have a windproof shell to keep out the wind, you shouldn't have any trouble keeping warm with a couple of thin layers.