Blocking is difficult to deal with. If you're covert, ok I'm not going to tell you not to substitute on difficult blocks because that's not the time to work on speech. Again, this is where a really good speech therapist can develop a plan and to help you gain control over what you say. That is of course, if you want to stop being a covert stuttering and fearing words. The hardest part may be finding a therapist who knows how to do this.