Well, a couple of things. I don’t mean to pile on, but another human being in the room with you at arms’ length is a really useful thing. Whether that’s an actual ‘teacher’ or just ‘a friend who plays guitar who is willing to act as a mentor in exchange for a good meal and a six-pack’ is up to you. It’s just that your tuning question could be answered very quickly if a fairly good player had your guitar in his hands for a few minutes. The answer could be that you’re not used to tuning, that the guitar needs new strings, that the tuning pegs are shot, that you haven’t sacrificed to Apollo recently…
Some things I can suggest on the intarwebs -
When playing chords, try starting out by strumming the chord just once, and then taking the full time left to shift to the next chord. Strum the next chord just once, then take the full time to shift to the next chord. In taking that time, figure out what is the best way to move the left hand from the first chord to the second chord. Is there a finger that stays on the same string but changes frets? Is there a finger that stays on the same fret but changes strings? Is there a finger that stays at the same fret on the same string in both chords? Those three things (guide fingers, parallel fingers, pivot fingers, respectively) help tremendously.
When strumming chords, the left hand isn’t doing much until the split second when it changes to the next chord. So, get your left hand used to where it needs to go in a longer period of time, and then as you get used to it, shorten that time.
Go back and forth between two chords, so that you truly get the notion that going from A to D or from D to A is like walking down the block to the next intersection.
I make my students do this exercise all time. Take those two chords, A and D. First, play them like this -
|: A | D 
|: O | O (In whole notes - count 1 2 3 4 and strum on the '1’s; take all four beats to shift the left hand.)
then |: d d | d d (In half notes - count 1 2 3 4 and strum on the '1’s and the '3’s; take all two beats to shift the left hand.)
then |: l l l l | l l l l (In quarter notes - count 1 2 3 4 and strum on each number; take the time available between ‘4’ and ‘1’ to shift the left hand.)
In terms of ‘the guitar part of a song’, I’m not entirely sure what you mean, so I may be way off in saying this. Still - if the song you are practicing was not performed as a solo guitar piece with strumming chords, then what you are likely working on is an arrangement. The recording may well have had guitar(s), bass, drums, keyboard, vocals, sax,… The recording also may well have used overdubs and/or the best available take. All this by way of saying that what you are playing may be chords that someone felt went with the original recording, but the guitar part on that recording may be a riff that the guitarist invented. You strumming the chords under the riff may not sound like it fits… To further complicate things, the chords you are reading may be simplified. Even if they’re exactly the right chords, the guitarist on the recording may be playing them in a different ‘voicing’, eg. the same chords higher up the neck so that a different note of the chord is on top.
I was working on Adele’s ‘Daydreamer’ with an adult beginner student last night, and this was the very issue that came up. In that particular case, the riff is very simple (but effective) and sounds much better than strumming the chords that a publisher printed above the staff.
So accept, for the moment, that it is an arrangement, a simplification of the original piece and try to hear how it relates. As you get better as a player, you can improve the arrangement to your tastes, until you get good enough that you’re doing your own arrangements.
And my apologies if I misunderstood your remark…
