Do not lose motivation or faith - you WILL get there
However, you might want to consider tweaking your plan in some ways to change it up. Not suggesting you switch to a totally new diet or anything. But try to incorporate a different variety of foods (even if the same calorie/protien/fat/etc breakdown). Change up type of workout, time of day, do more interval training?
And do some self-checking - in the 20% of time off plan, are you causing your problems? Or having a 50cal off plan day? Are you on top of water consumption? Any odd processed foods you are depending on? Does your colon feel like its getting good function? etc...
Most of all though - know that tenacity and patience will always break a plateau and just hang in there. The tweaking may just distract me more than it helps me sometimes