It's true, most trainers and fighters I know do not engage in sexual activity before a fight, some for as long as seven to fourteen days. When I train in both weights, cardio, and MMA, I do notice a big difference in my energy level and stamina. It's not a placebo effect since when you spar you can tell the difference in their speed, power, and reaction times without asking them about their sex lives in the ring.
There's definitely a science behind this, and I wish more professionals in the medical field would investigate this instead of being deterred by the stigma attached to medicine related to sex.
To OP: Yes, blowing your load too often can and will weaken you. You need to heed the previous posters' advice on replenishing what you lose by taking supplements and eating better. You will also need to cut back from those blow jobs if that doesn't work, it takes far longer for the body to build back its reserves than to lose them.