Listening to your stuff really quick, it definitely seems like that all your troubles are in mixing and arranging, not in how to use FL Studio. For starters, when you write your tunes, make sure there's a bass voice whenever you want it to sound full. It sounds like you have a melody and some mid voices, but the bass is supplied entirely by your kick. Having a bass voice (bass guitar, contrabass/double bass, or some synth that sounds cool in the lower registers) will /really/ make something sound a lot fuller.
Typically, the fewer mid/bass voices you have, the emptier your arrangement will sound. Moving lines will sound a lot thicker than non-moving lines. So listen to all your favorite artists and pay attention to how they layer their instruments to create textures. Oftentimes, there will be plenty of moving lines that aren't the melody, and there will almost always be a bass voice filling up the low end and supporting the whole song.
Next, when you mix, make sure everything is present. The first thing you want to get down is making balances right. So volume-wise, make sure vocals/melody > drums > everything else, assuming pop (especially electro-pop). Pay really close attention to to what's actually supposed to be the focus. Something that was the focus at one point might not be the focus anymore. (Like in your Winter Wrap Up remix, where you have the repeating vocal line. Twilight starts as the focal point, but then fades into the background as the WWU melody comes in. But you don't lower Twilight's volume, or introduce the new line at a higher volume. Then, you end up with a lot of vocals conflicting with each other even though they shouldn't.)
Now, past all the basics, you have a plethora of different effects to mess with to make a song sound better. The two biggest ones are compressors and EQ. Start with the EQ. Something that you should keep in mind is to
cut; don't boost! If something would sound better with a tiny bit of a boost here and there, that's fine. But for the most part, you want to cut offending frequencies from the sound. There are some guidelines to follow (like, highpass /everything/ as much as possible save for the bass, kick drum, etc.), but for the most part, this is entirely up to you to figure out. Mess with the EQ, figure out what frequencies sound bad, and then lower them. This one isn't too hard to get the hang of.
Then, compressors. They're really hard to learn, mostly because it's so hard to hear their effects at first.
Read this thread on them, 'cause it explains compressors far better than I can.I'll be making a video soon explaining basic mixing/mastering topics, so watch out for that! ^.^ </shameless self-plug>
