How to Create a Song.... Music like painting uses more standard components (like faces, hands, trees, rocks, houses in painting), these are building foundations that you essentially add in as details with their own improvisation (local randomness that fits in in general and memory to hold on to them for music or at least a good recording medium you can edit well.). Some of your building blocks are better, some are worse you find as you are writing your tune, the trick is to find and keep the best and omit or improve the rest. Like putting a puzzle together piece by piece you leave some of the blocks in or some out of your finished creation, and often you edit the blocks (usually of standard time length with some exceptions for creativity too).

More options are important for your song so you aren't limited to the same smaller motif even if it's classic. One way to add more options is to jam with other soundsmiths (they didn't know who they were!). Or you can use one instrument with a lot of range and ease of play so you have complete control of all aspects of your song and don't need the other input. A good way to have the complete control is to buy a keyboard with touch sensitive keys, a built in recorder and no clicks if you make a goof like on usual keyboards by way of direct connection. These also have software like cakewalk and you can get one for just e.g. about 100 at stores like Heartland.
Another option if  is to take an instrument like a guitar and retune the sound to create new songs that suprise you too so you're more inspired. As I say this has the advantage of innovation but the disadvantage of having to learn the song more completely from the start. And a hybrid method of just a retune of the higher strings is often optimal because you can play much of what you already know but in a sort of aurora of new sound. Then when you get tired of the new tuning after a few songs you can just go on to other tunings. (Good to save the old tuning, I just write it on the back of my (cheap) guitar on a postit with heavy invisible tape, later on you can come back to the old tuning for more depth yet. It's like getting 10 instruments at one usual value.)
Generally you go in cycles in your songs, e.g. you can start at a low medium keynote go higher and higher, then perhaps down on base side then return to keynote then rise with another block then cut in another key then back to keynote then up and finish the cycle this is a classic form for a song especially if you change the details of each part of each loop.
Standardization is important for playing acoustic instruments, that is you want your nails clipped to the same length, sitting up at the same angle to not bounce and hit other strings, light regular plucks, no water on your hands ect. I've often wondered if like the electric violin that's silent there couldn't be a good retrofit invention of add on teaching software and hardware that fits over your old instrument like say a woodwind or horn so you are taught how to play automatically the right way all the time for the fabulous real sounds and options you can only find with THE instruments. My mom says she thinks synth music is sort of limp and without all the natural sounds you hear with "real" music. If you like to hear the old LPs with real pops and crackles it's because you know more than just top 100. When super artists do digital art they always start with good old real drawing, no wonder, digital is more like an improver than a foundation. Ideas come first or computers would do all the creativity, right.
Put in all the notes when you practice. If you have a good enough instrument this will be no problem but if you can't afford a high end wage with a cheaper instrument press and pluck harder ect.
To strengthen your hands play say for five minutes then rest before you hand hurts much then rest and resume while flexing your hand in reverse. All your muscles need to flex both in and out to keep your reflexes and not damage them.
No sections in any of your songs where your hand is moslty too stressed to mostly easily play the finished song. Extensve times of your tune where you can't play without stress are without value. Even so brief reams where you learn it well and it doesn't throw the rest of your song are of value if it's merely uncomfortable while you practice because you have to go over it a lot to learn it but not when you play the finished song. If you use the Theory and Experiment Method (see the main page Method 3) to learn this realm of the song it's still often a valuable addition to your song achievable by no other math method!
When in doubt to refine the building block to say speed up or slow down or more or less in a yes or no form is of real value. For a tough realm, start at the start, learn some rest, learn more, rest from start on through, add more over time even if it takes a month. Easy realms of the song will go faster and this is a good morale boost till you've learned the whole tune. When doing the more difficult realms of the song concentrate on counting and more on what you know definitely while adding more of what you don't yet know by improving and improv. With counting each number you associate with each motif this makes it so you can practice it out of context with exercises to finish well and real good so the royal princess english are and is saved, just by picking random points of departure and then going over what follows it there in the song and with counting. For counting with a few exceptions since your timing is mostly standard of each section for symmetry ect.  I use a general to exact method, i.e. I have say 15 sections lettered A to O and because most of the timing is regular each of the 15 has say 1 to 4 subsets and so to find any part of the song and remember helps a lot to both find and say "the time coordinate" with each rep, say G4 or B2. Once your realms are mostly set and you are finishing up the tune say adding a bit of change to B3 or F2 (Like in music college I never got A's or  D+ always D major!) you merely go to that realm and do reps of what you want to remember to add in just there plus the B3, ect. Here it's good to improve your memory faster provided you're already are sure what you want to add in by finding the notes and do "small exercises" to fit right in. Say you want to remember F to F for A3. Here you can start with the start of A3 plus say reps of E to F three times with each rep while saying what you want to remember too! This "cartoon method" not only improves your memory, it also seems to be a real boost for creativity in general because it is a cartoon method. In truth this may add too much creativity, being the opposite of memory per se. So it's good to go in cycles of the cartoon rep method alternating with just the more usual method of improvements by the science (Method 3) and then reps to more gradually improve. If you try to improve too fast you're too unfamiliar, so it's good to go in cycles here. I generally go without written music and just count the general area of the song (say block 1 and block 2)  and not each not, this makes me faster to learn.