I found some of my personal training clients enjoy it more when they have exercise goals beside weight loss or injury prevention. It may help you, too, to stick to your training journal.
Why Pull Ups so hard?
Muscles have origin and insertion. Normally insertion of muscle moves towards the origin of the muscle during concentric phase (biceps curl, triceps extension). On the other hand, pull ups are the complete opposite. Insertion of lat has to pull its origin that makes pull ups so hard.
Do you want to learn it? You should mimic the movement.
Back in the day, I was doing gymnastics, and wanted to learn back tuck, back handspring and so on. I learned a lot about how to do them on Youtube, and what theory behind it. But once I decided to go to the gymnastics gym and talked to one of the gymnasts there. Proper process is always to mimic the movements. Otherwise, your body never learns how to do back handspring.
Assisted pull ups ( machine and resistance band)
Some gyms have a machine for pull ups. It can vary the weight, usually with 15lb increments. Unlike other machines, lower the number of weight, harder it is, because the less weights assist you.
Resistance band is a great alternative. Note that since resistance a band assists you, the closer you get to the top, the harder it will be because the band stretches less obviously.
Pull Ups Eccentric Focus
There are 3 ways to train any muscles. Concentric, eccentric and isometric. Pull Ups eccentric movement is the down phase. So what you can do is jump up to the top of the bar, holding the position, then slowly come down. Try that until your muscle gets fatigued. Make sure that you come slowly using muscles, and fight the gravity.
Example of eccentric pull ups here.
Learn Chin ups First
Chin ups are slightly easier than pull ups because with under hand grip, biceps muscles will turn on and will generate the power as well as lat. a semi-supinated grip that is palm facing each other, is also easier than over hand position.
Click here to learn in-home back muscle exercises