Don’t be embarrassed, you stammer, so do I. It will get better. This works for me… 100% of the time, but we are all different so it might not work for everyone but I never attempt to swop words, when I feel a block coming on, I simply concentrate on the word that is going to block and if I tap my foot (discreetly not stamping) 2 or 3 times whilst starting the word in my head and breathing in, by the 3rd tap I unblock and am able to say the word. I pause for no more than a second or two, I know this sounds a bit strange but it has given me back my speech back and I’ve using this technique for many years.