What you describe is not normal battery behaviour if the battery is not that old (for reference, Fairphone’s warranty for the battery spans 1 year, but depending on individual usage the batteries can perform well even longer).
A first trivial measure would be rebooting the phone to shutdown processes eating battery because they somehow hung themselves up.
Else there are general hints on improving battery life in the batteryguide.
You might consider looking into how to backup your important data, and then do a factory reset, which often helps in cases where the phone becomes slow and unresponsive …
… or, in case of an OS problem or you just wanting to be more thorough about it, completely wipe the phone and install from scratch (which a factory reset on the Fairphone 2 will not do) …
Then … Which OS are you running, and are you running a current OS version or an older one?
Since every major Android version upgrade so far includes battery life improvements, there are differences depending on which OS you run, and which version of it.
Fairphone’s own OSes are at Android 7 (Nougat) currently, which should have better battery life than Android 6 (Marshmallow) before or Android 5 (Lollipop) which the Fairphone 2 was originally shipped with back when it came out.
LineageOS on the Fairphone 2 is at Android 9 (Pie) currently and should have better battery life than Android 7, but it is not officially supported by Fairphone, it is a community effort.
Apart from that, Google Apps and services are known to eat battery compared to when you not have them.
Fairphone OS includes Google Apps and services, as most people want to have them.
Fairphone Open OS and LineageOS don’t include them by default.
If you want to consider this, here’s a tag for a start … livingwogoogle.