This is one of the most difficult sounds to speak, and to make matters worse, there are two different ways to say it! There are two types of voices: voiced and unvoiced. The "TH" sound is particularly difficult for French people to pronounce because it does not exist in their own language.
That the hardest sounds for children to learn are often the l, r, s, th, and z is probably not surprising to many parents, who regularly observe their children mispronouncing these sounds or avoiding words that use these letters. Typically, such behavior is completely normal for children.
“R” is the hardest single letter sound to master. Many English speakers never do. In British English, it's omitted entirely when it's in the middle or at the end of words, and many English speakers substitute a “W” sound which is actually very close.
Hard sounds are generally louder and make a sound that is more noticeable. Soft sounds are not as loud but can be more pleasant to the ear.
The first step is to find a rhythm that makes sense for your writing, and then stick with it when you write. If your writing has a lot of breaks, that might be why you’re having trouble following along. Make sure to build breaks in your sentence structure so they are not all one long sentence with no breaks.