What should I do about the decline in my devotion and my shortcomings in worship? And will this distance from God negate my previous good deeds?

The soul has periods of inclination and disinclination, and rarely does one find a person who maintains a single pace. The Messenger of Allah, peace and blessings be upon him, said: "Indeed, for every deed there is an eagerness, and for every eagerness there is a slackening. So, whoever's slackening is towards my Sunnah, he has been guided. And whoever's slackening is towards other than that, he has perished."

If, during your period of slackening and weakness in worship, you maintain the foundation of religion while avoiding sins, then you are upon good. However, if you completely cut yourself off from obedience, that is destruction.

What nullifies previous good deeds and invalidates actions is shirk (associating partners with Allah) and what is akin to it. Allah Almighty says: "If you associate others with Allah, your work will surely become worthless." So, if you did not commit anything considered disbelief during your periods of laziness, then hopefully your previous good deeds will not be lost.