Believe me, it's not hard at all to get into mixing and mastering, even if there's generally a billion options in specialized softwares, and that can be intimidating ^^

What software are you using ? There's a lot out there, free or not, and some are easier to get into.
But most of them can do the basics you need.
Still, I can give a few tips and advices. Even if I'm far from your composing level, I did learn mixing and mastering at school. ^^

First, try to switch from speakers to helmet while listening to your work. Some devices are really different from each other in the way they rendeer bass or high-pitched noises, and weird things can happen ^^ (Mix with a cheap helmet that doesn't render basses, then listen to your work in a studio with a normalized and full set of speakers ? Instant earthquake !)

Second, stick to the basics, that are :
Panoramics : left or right and all. A nice touch is to double a lead instrument and put them equally far from the middle (+30 and -30 for example) to get a nice plenty and surround effect ^^
Equalizers : or EQ for short. You can magnify an instrument with it, or completely turn it into something else for the ears, so don't go too far.
Compression : does wonder on bass and percussions, but take it with a grain of salt ; the mp3 format used everywhere does a heavy compression by default, and too much is too much.
Gain/volume : this is the easier part, but if you want to be normalized, try to put the highest volume of your audio between -3dB and 0dB on the master meter. So people won't have to turn up or down their speaker when they get to your music in their playlist ^^

One last tip : if your file after all mixing and mastering is heavier than before, you probably did it wrong ^^
Mastering is for polishing the audio, you don't add to it ^^

I hope I made myslf clear. English is not my main tongue, and I don't know how much you know about mixing and mastering, so tell me if there's something you don't understand, I'll try to clarify ^^

I'm having a hard time wording what I'm mostly unsatisfied with, but I'll try:

As for audio devices, I'm using the following, sorted by frequency of use (high to low):
- Creative Gigaworks T20 Series II multimedia spreaker system;
- Shure SRH940 Monitoring Headphones;
- Sennheiser CX 3.00 in-ear earphones.

As I'm at a total loss about what FX software to use, all I use are plugins provided by my DAW (FL Studio 12 in this case). I often don't go farther than using EQ and an occasional Compressor (for percussion mainly), running a mastering tool in the Master send (i.e. FL's Maximus).

I think that the main problem is that I often 'have no idea what I'm doing' and either end up using templates or screwing around until it sounds reasonable (e.g.: I ran a compressor set on a brickwall preset on the brass and the percussion (excluding the cymbals) in my last track) being unaware whether I'm doing something wrong or not (perhaps I end up using tons of unnecessary FX and RAM). This counts mainly for compressors, maximizers and limiters. So maybe it isn't necessarily about what to use, but rather how to use, but I'm not sure.

In general I guess I'm not satisfied about the tone and perhaps the inconsistency in sound as well, e.g.:
- It sounds muddy or synthetic;
- It sounds ' flat', i.e. accents/ nuances have little or no power;
- It doesn't sound like a one track - rather like various tracks recorded in various environments mashed together.

Anyways, thank you very much for taking your time to respond to my journal! I appreciate your tips/ advice a lot and I surely will take this in consideration upon creating a new track~
~Silent

FL Studio is a good software, with amazing samples and plugins. ^^
I once tried to learn how to use it, but I didn't like how the UI worked, so I ended up sticking with good old Protools.

Using the templates is absolutely fine, and helpful too if some small tweaks are enough to get to what you want for your sound.
Even the pros use templates.
(Lazy ones tend to ONLY use templates though ^^)

One question : do you mix your tracks as MIDI ones, with the sampler running all along ? Or do you convert all your tracks to uncompressed audio files (.wav for example) to do your mix and mastering ?
I don't know remember enough about FL Studio to know if it's easier to mix audio files or stick to the MIDI.
If you have troubles with your RAM, going for the audio files is the way to go, but you have to be sure about your recordings ^^

- If it sounds muddy or synthetic, it's probably your sampler. No virtual instrument can match the clarity of a real instrument recorded with a good microphone after all ^^ but you can try to tweak the medium/high-frequencies with your EQ to gain some clarity. It may cause the sound to become "cheap" though.
- The flatness and no more accent might be caused by your compressor. Compressors put all frequencies at same level basically, giving a nice boost to the audio, but it flattens everything in the process, thus the nuances and accents losing ground.
- That third point sounds like a mixing problem. And it's an art it itself, to make many tracks become one for the ears. Practice makes perfect ^^
Have you tried to add fade-in and fade-out to the beginning and the end of your tracks respectively ? Even really short ones (less than 0.1s) can help to ease the coming and going of each track, thus making everything smoother to the ears ^^

I'll probably stick to using FL as it has been my go-to DAW ever since I started composing!

Usually, I don't need that much FX as I try to level out quite some instruments on the go. However, it might result in an unbalanced track, it's something I can't really explain but I feel like it makes my tracks rather unconvincing (in comparison to more professional composers on here and out there).

Usually, I can limit my RAM usage to the point I still can record the MIDI. Only if my sampler fucks up (rarely now since I got a SSD), I resort to recording the audio.

- What I believe as well is that tweaking up some frequencies might cause interference with frequencies of other instruments, causing this muddy-sounding well... Sound when both instruments are played together, for example.

As for the rest, I assume it is a matter of practise, something I can do more and more since I finally upgraded my arsenal of audio devices (I must say, I have been composing on mainly cheap earphones for years)