Your diagnosis is that you have what speech therapists call dyslalia which means that proper tongue movements are not achieved to pronounce the required sound. Although tongue structure palys some role in this issue, it is generally a tied-tongue which interferes with its adequate movement - this means that the ligament connecting the base of the mouth with the bottom of the tongue (called frenulum linguae) is sometimes short and needs to be released. This 15 minute surgery called z-plasty is best done by a plastic surgeon since the job is done with greater finesse and no residual skin is left. This surgery does not do anything to the tongue - only to the ligament below it. It is difficult to say exactly what is causing your speech problem without seeing you personally but this above info is based on what you have written about your problem. Finally, let me assure you that it can easily be cured with the correct speech therapy.