What should one do who suffers from semi-permanent flatulence before and during prayer, and loses concentration due to trying to hold it in, knowing that it is not a permanent incontinence?

It appears you are experiencing some waswasa (whispers/obsessive thoughts). The cure for this is to disregard and not pay attention to doubts about passing gas. Build upon the original state, which is that nothing has exited. Do not prolong your stay in the restroom; exit immediately upon finishing your need. If you find a period of purity for prayer, wait for it. However, if the impurity is continuous such that you do not find a period of purity, then your ruling is that of one afflicted with perpetual impurity (sahib al-salas). Perform ablution after the prayer time begins and pray the obligatory and supererogatory prayers.