It has now dawned upon me that English will take me a lifetime to learn. I want to tell my kids how much I regret not having begun it earlier, and warn them not fritter away their precious time on nonsense, but I know I can't bring myself to do this. What I can do is talk with them as much as possible in a friendly way. And this, I always do. That way, I can feel them getting better. Formerly, I would push them and it didn't work. So now, I opt for slow, but steady and happy, improvement. I am mostly just giving you stylistic suggestions. Your only actual mistakes are: first sentence: The role of "only" is not clear. "Take" is not conjugated correctly. second sentence: "on", not "for". "to learn" is not wrong, but "having learned" is better. You are right. Learning a language is a lifetime project. That is true not only with foreign languages, but with your own native language as well. I will never stop learning new things about English.

Now I realize that only English takes me a lifetime to learn. I want to tell my kids how I regret not learning it sooner or to push them not to waste time on other nonsense but I know I can't. What I can do is talking with them as much as possible in a friendly way and I always do it. I can feel they are getting better that way. Actually, I used to push them and it didn't work / it never worked. Now, I choose to steadily and happily improve things even if it is slow.

Now I realize that only English takes (singular) me a lifetime to learn. I want to tell my kids about how I regret not learning it sooner or to push them not to waste their time on other nonsense/trivialities but I know I can't. What I can do is to talk with them as much as possible in a friendly way. And I always do this. I can feel they are getting better that way. Actually, I used to push them and it didn't work. Now, I choose to improve things even when it is slow but steady and happy.