How can one who is committed to worship and far from sins overcome their fear of backsliding and returning to sins, ensure continuation in obedience, and at the same time balance between the love of Allah and the fear of Him so as not to feel secure from Allah's plan?

Your fear of not continuing in obedience and falling into sins is a good thing, but it should be a motivation for more obedience and staying away from disobedience. What helps you with this is supplication and humble entreaty to Allah for steadfastness, and the companionship of the righteous. To strengthen your fear of Allah, reflect upon His Most Beautiful Names that denote His greatness and the severity of His punishment, while also hoping for His mercy. It is better for hope to prevail during illness and death, and fear during health. Having a good opinion of Allah is required, but it should not lead to feeling safe from Allah's plan.