In addition to what your doctor said-- no caffeine after 3 PM. If that's not enough, then try stopping caffeine at noon or even 10AM. Same thing with spicy foods.
Nothing exciting 2 hours before bedtime. So after your walk (a good habit to get into anyway ), no loud or fast music, no exciting or horror movies, and stop looking at the news. If you have a TV in your room, get it out of your room if you can.
Keep your phone out of your bedroom if you can. If you need it (for an alarm, work, etc.), then set it to vibrate and turn it over so you don't see notifications coming in. Try to stop all phone activity an hour before bedtime.
Use dark, light blocking curtains (you can get them on Amazon or have them made). If you wake up with backaches then you need a new mattress. Spend some money-- it really does make a difference. If your pillow doesn't give you enough support, replace it.
Keep your bedroom a little cooler than you might want. Shut windows to keep the noises of your city or town out as much as possible. If you need it, put on a fan or other droning sound to create white noise. In the summer, we use a fan. In the winter, we use a humidifier. They both work and of course they also have a second purpose so it works out really well.
Write down anything you need to remember, whether on paper or your phone, so you're not staying up and fretting over things like that.
Maintain a regular bedtime as well as you can. Weekends are going to be different, but during the week, set a reminder to go to bed at a specific time (say, 10PM), and set a second reminder for an hour earlier to remind yourself to start getting ready for bed (shower if you do that at night, etc.).
It personally helps me to lay out my clothes for the week but if you feel that's excessive or you don't have the room, then just lay out your clothes for the next day. Again, it's another thing you won't stew about.
I also do a kind of calming ritual. It's very basic, just thinking about things that I like. You can do something like this, or pray if you're religious. Just, it's something that is telling your brain, it's time to go to sleep.
If you do get up during the night, don't check your phone or turn on the TV. Get back into bed. But don't turn bedtime into frustration. If you can't sleep after trying for an hour, then get up-- and if it's more than an hour before you have to wake up, try calming yourself again. If you really, really can't sleep, it's okay. That happens.
Keep your bed for two purposes only: sleeping and sex. This means, don't work in bed, eat in bed, watch TV, etc. Don't associate your bed with anything but satisfaction and sleep.
I know that this is a lot, but it'll become second nature.
PS your English is great and very easy to understand.