I recommend a detailed study of Sri Patanjali Maharshi's Yoga Sutras. His lay out is the curriculum for yoga. It is classic and final. Nobody can improve it. At first this book might seem difficult reading but if you are patience, you will get much insight about Yoga.
I was doing pranayama from youtube (including the videos with the well known Indian guru who teaches it - cant remember his name, ramdev?), then I bought a book on Pranayama:
https://www.amazon.co.uk/Pranayama-Breath-Yoga-Gregor-Maehle/dp/0977512622
Which is SOOOOOOO much better. It gives the fundamentals and explains everything and gives so many small important points that I now look at youtube videos and am amazed at how basic they are and miss so much.
Meditation Expertise is the most advanced and detailed translation of the Patanjali Yoga Sutras I have read.
Meditation Expertise (Commentaries) https://smile.amazon.com/dp/0981933254/ref=cm_sw_r_apan_glt_fabc_38FX4EFMM070WBB8VR8Q
There are suggestions out there and here in these comments that are good starting points. Your body is a laboratory. Not only that, your body is YOUR laboratory. You can use the suggestions as a starting point. I used Rosen's book https://www.amazon.com/Yoga-Breath-Step-Step-Pranayama-ebook/dp/B00ATMWK6G/ref=sr_1_1?dchild=1&keywords=richard+rosen+pranayama&qid=1596653971&sr=8-1 to get my starting point. He has you explore how you feel with various patterns. Then after a few months of daily practice I discovered it was more important to emphasize Inhale Ujayii III to wake myself up as a prep for Anuloma and Exhale Ujayii II if I'm keyed up from something I was doing. Kapalbhata is a good substitute for me for Inhale hold. My laboratory. Happy hunting!
1:4:2 is final and at this Ratio you can increase time of inhaling, retention and exhaling . At this ratio you can increase gradually For example 2 sec inhale, 8 sec retention & 4 seconds exhale.....
3 sec inhaling, 12 sec retention, 6 sec exhaling.... Similarly you can go to 16 sec inhaling, 64 sec, retention & 32 sec exhaling .
THIS SHOULD NOT BE DONE FORCIBLY WHICH MAY CAUSE UNRECOVERABLE DAMAGES TO HEALTH AND HEART ATTACK. THESE SHOULD BE PRACTISED UNDER THE DIRECT GUIDANCE OF AN EXPERT ONLY. NOT THROUGH ONLINE VIDEOS, OR BOOKS.
I WILL GIVE AMAZON LINK TO A PRACTICAL & GOOD REFERENCE BOOK WHICH SHOULD BE USED AS REFERENCE ONLY.
Why can't you get through it? Have you tried an app? if you're on android this one is awesome. So much control and structure. You can gradually work yourself up if the problem is what you're practicing is too hard...if it's patience and keeping structure over a long period of time, the app is amazing. https://play.google.com/store/apps/details?id=com.abdula.pranabreath&hl=en