@Markus, I'm very glad some of my advice has helped you become more flexible and your back feel better.
Regarding the pain you find when stretching, consider if you asked the same thing about lifting, e.g., that you feel pain when performing a certain lift. My advice in either case would be the same - first, consider seeing a doctor to determine if the pain's cause is something other than exercise. Second, just as it's possible for your technique or lack of proper technique to result in a painful and unsuccessful lift, so the same is true with stretching. I have no doubt that I could cause myself pain when working on a split if I didn't do it as instructed.
Just as you can get hurt pulling a deadlift or doing a swing that's too heavy, so you can feel pain when stretching if you're trying to stretch more than you're ready to do, and it's impossible to say without seeing if you're ready for the depth of split you're attempting or not, but it sounds like not. We know, e.g., that even a light swing, without proper bracing, can make your lower back sore; a split's no different except that the technical points aren't the same.
Wish I could tell you more but that's the best I can do.
-S-
Regarding the pain you find when stretching, consider if you asked the same thing about lifting, e.g., that you feel pain when performing a certain lift. My advice in either case would be the same - first, consider seeing a doctor to determine if the pain's cause is something other than exercise. Second, just as it's possible for your technique or lack of proper technique to result in a painful and unsuccessful lift, so the same is true with stretching. I have no doubt that I could cause myself pain when working on a split if I didn't do it as instructed.
Just as you can get hurt pulling a deadlift or doing a swing that's too heavy, so you can feel pain when stretching if you're trying to stretch more than you're ready to do, and it's impossible to say without seeing if you're ready for the depth of split you're attempting or not, but it sounds like not. We know, e.g., that even a light swing, without proper bracing, can make your lower back sore; a split's no different except that the technical points aren't the same.
Wish I could tell you more but that's the best I can do.
-S-