Think of learning the guitar like chipping away at a mountain with a spoon. It literally takes years to develop technique - and that technique doesn't just one day pop into perfection. It goes thru levels of forward and backwards progress. You have high expectations for yourself, and you're getting frustrated because you're not reaching your goals fast enough - that's natural because people expect quick results - we don't naturally think in terms of months or years to achieve goals - we're more in the weeks camp. Everyone has expectations, but realistically speaking - you need to be thinking in terms of years, not months, especially for something like guitar.
The mind is willing, and the body has the physical strength, but coordination and mental aptitude rely on nerve-endings and fast-muscle tissue - those don't just develop overnight. A brand-new exercise, using areas of the mind / nervous system that your body has never done before takes a minimum of 2 weeks of regular observance just for the neural pathways for those signals to form. After that is the fine-tuning phase which takes a life time. If you've ever picked up something for the first time, and it seemed awkward at first, that's because you haven't developed the neural pathways for it yet.
Fine-tuning something as precise as alternate picking at a very fast speed is not something the body can achieve in a short period of time. Sure physically you can make the pick go up and down, but transitioning from string to string requires a lot of fine-tuned motor and neural pathways to fire perfectly to avoid the first string on the way over to the second string. Don't downplay the physics in that exercise. It really is an amazing task that the human hand has to achieve at high speeds.
Anyways - yes we all get frustrated. There are huge learning curve jags in the rise to perfection. The first jag is first starting out - you have 2 weeks of learning how to just hold the pick and it not feel completely awkward - those are your neuron pathways forming. Then the learning curve speeds up for a while - then you hit the base of the mountain - that's the threshold of your speed and coordination - now for the climb up the mountain. The climb up the mountain is a guaranteed number of years of pain and suffering.
Consider this: No athlete picks up a ball and becomes a major league player in less than 5 years. It just doesn't happen. Guitar is no less rigorous a sport - not physically - but agility wise it really is. Best thing you can do is stop thinking of your practice as a short-term "reach the major league" kind of exercise. Instead, think of it in terms of a life-long activity that gets better with age. As a guitarist, you have something professional athletes don't have - longevity. Take advantage of it. Athletes peeeter out at age 30 - 40 is much rarer. Les Paul and BB King --hahaha. you got an extra 40 years to get good man. Stop racing. All the hot rods on guitar started at age 9, 11, etc. They weren't famous til they hit 20. That's 9 years of practice to get fast. Just keep that in mind.
Christian A.
Let me add this - I also experienced having alternate picking down really well once when I first started playing when I was about 20. I took a 15-20 year haiatus because I got married and suddenly busy lol. - I picked up my guitar again about 2 years ago, and was able to AP just fine - for about a month. Then I tried doing scales with AP and I could not for the life of me do it faster than about 50 bpm I kid you not. I was fumbling around - it was really frustrating so yeah I hear you on that front. I stuck with it and within a few months I'm back up to doing full 2 octave scales fairly cleanly at a good pace. So patience and perserverence won. I'm sure it will with you too. Just keep at it and don't toss your guitar out the window!