@arsenalfanrichiidk about you, but i had a different problem: using too much time in the opening. ;-)
seriously: It is a very interesting observation of yours, that your attention/interest/focus is vanishing while waiting for the opponent to move. Many things, you could try here (choose an idea, that suits you, must not be one listed here):
- try to understand, what exactly is happening: what kind of external or internal behavior is triggering this movement in you?
- relax physically, maybe also mentally, possibly by including little excercises to make it easier (walking being just an example)
- raise interest and focus by imagining future developments in this game and answering questions that would be relevant in the future (what trades would help, which one should be avoided - as an example)
- keep in touch with the reality, that our conscious minds are only a tool to solve problems, best applied, when the latter appear in your focus. But best play needs other parts of the brain to be trained and used ("default mode network" is how neurologists would call it)
- allow your perception to include observations, that may look unrelated to chess - at first glance - like your attitude, emotions, feelings towards opponent, your assumptions about his motives, you own breath, heartbeat, things like that (do not forget to eat and drink if need be).
- and do not expect immediate success, some things need time to develop (a.k.a. build new circuitry in the brain)
- By now, you may have some ideas of your own (myself used to say: It doesnt matter, HOW you are getting there, as long as you GET there. I preferred making love to my wife while getting there ;-) )