Faulty spark plugs and fuel system issues could be at play, too. For example, moisture in your fuel lines could be inhibiting your start function.
If possible, try keeping your battery (and your car) warmer over the cold winter months. A battery blanket is a good option, or you could park somewhere indoors just for the coldest bit of the year. If that doesn’t help, turn your attention to the fuel system.
Don’t feel bad if you can’t resolve this issue on your own. You can always ask a trusted mechanic to check out your 325i or bring your car to a BMW dealership.