People ask this question a lot:
Will I ever be able to play these tunes at session speed?
My suggestion? Ask this instead: "How can I get my tunes up to session speed?"
I've said it before but there's no harm in saying it again: The only way to learn music is through mind-numbing repetition...mind-numbing repetition...mind-numbing repetition...mind-numbing repetition...mind-numbing repetition...mind-numbing repetition...mind-numbing repetition...mind-numbing repetition...mind-numbing repetition...
Hey, what just happened? All that repetition, and our minds went numb, right? We didn't even read each repeat; just skipped ahead to this paragraph.
Hm. Perhaps a better approach is MINDFUL repetition, but understand this: as soon as we understand how to repeat mindfully, the next step is to lose our minds.
Some call it muscle memory: playing a tune over and over so many times that eventually your fingers know what to do without your mind getting in the way. (Remember that before you start getting all repetitive in your practice, you must warm up first and then take breaks, to minimize bodily damage and prevent repetitive stress injuries.)
Once you really know what you're doing, your mind can then go on vacation again. Or at least, it can stop paying attention and start having fun. Back when I was working hard on jazz, I found that I could take a pattern around 12 keys at lightning speed with no error. When I hit those beautiful moments, it felt like my body was working and my mind was watching—but the problem was, it was always watching. As soon as the mind said, "Hey! Check that out! Look what you're doing!" I lost it.
So, the next step in mindful repetition is to get your mind out of the way. As soon as you think about what you are doing, you're going to lose it. That's why we can play a tune perfectly a thousand times in practice, but as soon as we get on stage or bring it out at a session, and think about something—"God, I hope I don't mess up," or "Shoot, I hope I remember this tune," or, "That guy on the spoons is bugging the heck out of me"—we mess it up.
What to do? Get your mind out of the way. But how? I don't know. Maybe telling yourself to "stop thinking so much" will work for you. People try everything. They tell themselves "be here now." They buy sandalwood incense and sit on a round cushion for 20 minutes a day. They smoke a lot of pot. They spend years determining the perfect balance of Guinness-to-sobriety that loosens them enough to play well but not be sloppy. They hire an exorcist to rid themselves of the voice of their mother, who left them too long in the crib. They develop a practical "focus on the task at hand, dammit" attitude. Or, they suffer with the great majority of us and live with the fact that sometimes we play beautifully, and sometimes we flub it up so badly that we wonder why we ever thought we could be a half decent musician. Then, we suck it up and keep trying.
Here's a different way of thinking that might reframe your approach.
I recently heard that there's science out there that supports a certain kind of "muscle intelligence."
Ever play that game with the dollar bill? You put your thumb and index finger in a relaxed claw, and someone else holds a dollar bill vertically above your hand and drops it, without warning, and you're supposed to try to catch it. Most people don't have quick enough reflexes to close their finger and thumb in time to catch the dropping bill.
On a similar note, I recently heard about a study of brain function and tennis players' serves—that the speed of a professional male tennis player's serve is so fast that no human brain can possibly respond on time. It takes time for the brain to perceive the shot, determine its direction, tell the body to move to that place, then tell it to move the arm and respond. Science says that the speed of impulses in the brain, even for the fastest people, is not as fast as a professional tennis player's serve.
Yet, other professional tennis players CAN respond to these serves, and the scientists felt that it was more than just anticipation that made their timely response possible. They theorized that there was something else at work: the spinal cord. They think it has an intelligence of its own that can trigger response.
That can be called muscular intelligence. That is, that the body knows what to do, even when the mind can't keep up.
On a musical level, that means knowing a tune so well internally that your fingers are able to play more quickly than your mind can perceive the individual notes. Sometimes this gets referred to as muscular memory, but this muscular intelligence idea is making more sense at the moment.
So... how do you do it?
1) Get the damn tune in your head. Leave the instrument aside and sing that tune. Can you sing it, without the instrument? Don't pick up the instrument til you know what every note should be.
2) Know where each note is, intellectually. Sing that tune this time slowly, and visualize each note on your instrument as you play... think of each letter name, if you want. For added challenge, do it with a metronome on, or while you're doing something that has regular rhythm, like swimming, walking, or running.
3) Learn the tune, slowly. One. Phrase. At. A. Time.
4) Practice it, a lot. But don't practice ANY mistakes. Identify the spots that are tripping you up, and isolate those until you can play them perfectly... even if it's just a little subsection of three notes.
5) Julie Andrews was wrong. Sometimes the middle is a very good place to start. Start your practice session on the B part. Start your session on the last phrase of the tune transitioning in to the repeat. Start and maybe even end your session with just that little part that's been driving you nuts.
6) Work with a metronome. It forces you to play in time. If you just can't use a metronome, then get someone who can play the tune to record it for you at a variety of speeds—WITH a metronome clicking away in the background, and play along.
7) Buy Roni Music's The Amazing Slow Downer software, and use it. You can import any tune in the world into it, then slow it down or speed it up without losing the original pitch. You can also loop small sections. It's brilliant. Use it to learn every tune off your favorite musician's album, and you'll save yourself a LOT of money on music lessons. Just visit your teacher every and then to identify and help you fix problems you didn't realize you had.
8) Record yourself. It's horrifying, but it will teach you a lot.
All of these things will help you to get your tunes up to speed, but unfortunately, they'll all take time. Musical advancement sometimes happens at a slow and regular pace, and sometimes we move in leaps and bounds. Either way, often we simply plateau. Being fully comfortable on the plateaus, and continuing to practice there, may be the wisest place to be, as in that relaxation, you're storing up energy for the next leg of the climb.
Whatever you do, keep at it and don't give up. It's always darkest before the dawn.