What should I do to strengthen my frail body due to a stomach bacterium? What is the ruling on increasing meals for fear of extravagance? Is there any prophetic recommendation for strengthening the body?

Not acting on the doctor's advice is a form of extravagance; rather, it is taking the lawful means. You should ignore obsessive thoughts (waswas).

There are ways to strengthen the body, such as exercise. Among the most important of these is consistently engaging in dhikr (remembrance of Allah), due to its effect on increasing strength, as Ibn al-Qayyim mentioned from Shaykh al-Islam Ibn Taymiyyah. He also mentioned the Prophet's (peace and blessings be upon him) teaching of Tasbeeh (SubhanAllah, Alhamdulillah, Allahu Akbar) to Fatimah and Ali before sleep to help them with their work, and that whoever perseveres in it will find a strength that makes them independent of a servant. He also mentioned an account that the bearers of the Throne carried it by saying "La hawla wa la quwwata illa billah" (There is no power nor strength except with Allah) when they found its carrying difficult. This phrase has an amazing effect in handling difficult tasks and enduring hardships.