Don't try to force yourself to read for hours or jump straight to longer sessions thinking willpower will fix it, because all you'll do is associate reading with frustration and headaches. Also don't ignore if something's changed in your routine (more screen time, less sleep, stress) because that really affects your ability to concentrate.
What works better is to actually start small: 10-15 minutes in a distraction-free space, phone nowhere near you, no background noise if that's an issue for you. Some people find it helpful to read on paper instead of a screen if your head hurts, or to change the lighting. Try reading at a time of day when you normally have more energy (most people do better in the morning than at night when you're already tired). If the book you're reading bores you, switch to a different one without guilt; sometimes the problem is that specific book, not your concentration skills.
Increase the time gradually, maybe 5 minutes more each week, but without pressure. If you notice headaches sticking around for a while after reading, it might be worth looking into other things (eyesight, posture, how much screen time you're getting in general). And be realistic about your timeline: finishing a novel before the end of the year is fine, but it's not a race. The point is to get back into the habit without it becoming a constant battle with yourself.