Unfortunately for you, you fit almost exactly the pattern of people that German immigration will be concerned about. You have no job; you have a relationship which is purely long distance; you seem to have few ties to your home country; you come from a country that many people try to leave and migrate to Europe. You look to them very like the kind of person who will come to Germany and then fail to leave. It unfortunately doesn't matter whether you are that kind of person, if you look like that kind of person they will deny you a visa. Simply promising to abide by the rules will not affect your application.
The most problematic item is your relationship. A five year relationship where you have never met the other person is unusual, especially if you were both sixteen when it started. Regardless of whether it is a genuine relationship, it sounds fake to the immigration officials, and that will likely cause them to refuse you a visa.
Your best immediate way of solving this problem is to abandon your plans to come to Germany, and instead meet with your girlfriend and/or her parents in Tunisia. German citizens will have no problem visiting Tunisia, and this will let your girlfriend and her parents see you in your home, and meet some of your family also. Doing this not only removes the need for you to travel to Germany, but also adds credibility to your relationship with your girlfriend, which will help future applications. Meeting her in some other country, such as Turkey, would work as well.
Continuing to apply after having been turned down, without a significant change in your circumstances, is only going to make matters worse for you. You need to do something to demonstrate that your relationship is genuine and that you have ties to your home country.
