I take both Further Maths and Psychology.
For me, the worst part of further maths is the option modules my college picks - but that's it, it certainly requires effort, but if you enjoy maths, further maths is a lot of fun!
For me the hardest parts of psychology are the volume of content (it's a very content heavy subject), and the essays - but I've never been great at essay writing as a whole, I'm much more of a maths/science person.
Doing Econ with Psych would probably be quite good because you'll develop transferable skills that will probably make both a lot easier.
There's a lot of options for both - but some schools have specific options they always pick and that you'll be taught. For Psychology when it comes to the actual exam, you'll be able to pick which ever options you want (though you may have to teach yourself if you prefer a module that you aren't being taught in class), but unless you get your schools to sign you up for the different further maths options (different papers), you'll don't have much of a choice.
Definitely have a look at textbooks and see if you like the topics being covered!