One Sound at a Time

In order to deal with problem 1 as mentioned in the previous post, the speaker of English as a second language first has to identify what sound they are having trouble with but they don't have to reinvent the wheel. You are not the first speaker of your first language who has had to learn English. Many have gone before you.The common errors made by people whose mother tongue is the same as yours are public knowledge.
There is a fabulous website called the Speech Accent Archive where they have samples of people all over the world reading a short passage in English and they analyze the departures from General American English. Using this, you can find out what errors you may be making.
Ask a native English speaker to help you identify one sound to work on. It could be "TH" for example. Then go to YouTube. they have little videos that will tell you where to put your tongue and lips to produce this sound. This is your "Sound of the Week". Work on one sound at a time.
Watch TV or listen to the radio just listening for this one sound. Go to places where you can hear English speakers.
Find a native English speaker and say "Am I producing this sound correctly?" Most people will be happy to help you as long as you give them a specific sound to listen for. If you just ask them to tell you when you make a mistake in English pronunciation, they may not be able to figure out exactly where you are going wrong or they may be unwilling to "criticize" you.
Next, google on "Tongue Twisters TH" You will be able to find lots of challenging tongue twisters. Practice, practice, practice.
Once you have trained yourself to accurately produce one troublesome sound, move on to the next. You will find that if you are able to accurately produce one or two sounds that you were having trouble with, you will have made great progress.