So basically an issue I'm having currently is actually playing/practicing songs to a metronome. I'm very comfortable with playing sweep picking/alternate picking etc. exercises to a click. But usually those are straight 16ths or 16th triplets all the way through, it's all one note grouping over and over. My trouble comes when I try to learn a song, or an actual solo, I just can't play it to a click. Something about switching between playing several different note groupings to a click is really unnatural and odd to me. 

For example, currently I'm learning a chunk of The Dark Eternal Night by Dream Theater, because I just got my first 8 and wanted some songs to help adjust to it. I can play the intro riff fine, but the tab I'm using says that the first bar is played as a group of three, a group of two, and two groups of four. For the life of me I can't play that to a click, because it not being in one subdivision messes with my head. Is there some way to adjust or practice this? I feel like this is a basic skill that I'm lacking, and it really does seem to hold me back. So if anyone has any timing exercises, or resources they can point me to to help with this, it would be greatly appreciated. Anyway, sorry about the rambling in this post haha. And thanks in advance for any answers, I appreciate it.

Here's the Guitar Pro tab for the song that I used as an example, if that helps at all: https://tabs.ultimate-guitar.com/d/dream_theater/dark_eternal_night_ver2_guitar_pro.htm
This doesn't really fix the problem but it might be a workaround depending on how you like to learn.

When I learn songs, even if I am slowing the song down using Guitar Pro to learn complex parts, I always find it easier to learn it to the drums rather than to a click. I can't view the tab right now but I know the song in question and rhythmically that intro riff isn't exactly complex, so assuming you know how it should sound I don't see why it would be too hard to learn.

One exercise I can think of that might help you practice different subdivisions is to make a quick rhythmic pattern on Guitar Pro. Set your bar to 4/4. Have one bar playing straight 8ths, and the next bar playing 8th note triplets. Loop it and practice this with the click enabled. The click will effectively create a polyrhythm with the triplets, and if I understand you correctly that is where you are having trouble mentally.
Something that might help is to go back and listen to the song, and tap along with it where the metronome would be.  Get a good feel for where the beats are when you're listening to the song and that should help clear up where the notes fit with the metronome in your head so you can get that when you're playing it.
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Random3 It probably isn't too complex, like you said, but I seem to have this issue with most songs. I can play an exercise that's consistent sextuplets just fine, but the second you give me a song that moves away from consistent groupings like that, playing it properly on time to a click just seems impossible for some reason. For whatever reason it's just difficult for me to think "Okay, so in this section I've got to play a triplet, a set of 8th notes, and then two 16ths". Maybe it's a case of over thinking, but nailing the accents and getting that in time is difficult for me. I'll definitely try that thing you said about learning it to the drums though, as well as the neat exercise you recommended. Thanks.

reverb66 That's probably a solid recommendation. This'll probably be a skill to develop, so working on simple songs to develop the fundamentals may be necessary first I suppose. Thanks, I'll give that a go.

Zaphod_Beeblebr Alright, I'll be sure to do that. I'm beginning to think that I'm seriously lacking in the department of basic musical timing haha, so hopefully this sort of thing will clear it up for me. Thanks man.
Have you tried switching up note lengths when practicing? E.g. 4 bars of quarter notes, then 4 of 16ths, the 4 of triplets, then 4 of 8ths, etc.... You need to be able to switch between different rhythms on the fly. Once you can do that, or if you already can, then I would try the one or more of the following:

1. Count out loud as you play. This is tough to count and play, but once you crack it, man it makes life easier.
2. Hum, sing, or clap the riff/solo over the metronome to familiarize yourself with the rhythm before moving to the guitar.
3. Double the speed of the metronome so you have more clicks and thus more guidance, and then half it when you've figured it out.

Remember a note will always land on a beat, so it will fit the metronome. Just gotta figure out the rhythm.
gweddle.nz Sorry for the very late reply. That's not something that I've really worked on. Clearly that's something that I should work on. I'll take all of those tips into account. I never considered that I'd have to go back and work on basic timing while playing songs, but better late than never. Thanks.