Do I need to dig deeper into the theory? I’m thinking of Sam Harris’s Wake Up, or resuming a book that helped my when I started. It’s getting easier over time to get distracted when meditating, and should be the opposite.

Emily P.
You will never get your mind to stop being distracted, what often happens after a short while, is that we as human beings have a harder time paying attention to things we are familiar with, which means there will be a period of time where it becomes harder, as you are not as interested subconsciously as when you started, you will have to continue without judging yourself too much until the point where you do it because you enjoy it, not that it should help, as that is when your brain is starting to get rewired, most likely another 1-2 months.
Also, often our increased expectations for our practice, in what makes it harder for us, see practising meditation as always being a beginner and it will most likely help.
However, please dig deeper into the theory and you will find the same ideas, but maybe presented in different ways that resonate better with your situation and that increases your understanding thereof.

Clara Z.
It sounds a bit like you're overthinking it. Try Eckhart Tolle's "The power of now" and maybe learn a new way of meditating.

Mikkel W.
It comes with consistent effort and practice. I’ve learned to keep things simple and don’t over analyze or complicate things. Meditation is quite simple. Meditation is contemplation, focused concentration, deep thought, asking Infinite Intelligence questions, visualization, getting into feeling states, being patient and allowing the mind to calm down, being aware but detached, self realization, becoming one with higher self as being the higher self, introspection, pondering, quiet time, reflecting, self examination, going into the mind and mindset, awakening the awareness and maintaining that awakened state, broadening the awareness, connecting as energy, working with energy, conditioning energy, so on and so forth. When we sit and think about what we are thankful for and feel that feeling, that is meditation. Walking meditation is focused deep thought emotional feeling and information gathering meditation. It’s ok to get distracted, just bring yourself back when your awareness awakens to and realizes it has fallen asleep and strayed. Keeping the awareness awake takes practice and effort. It requires a consistent effort and practice all day long in all activities. When that is practiced, meditation without getting distracted gets easier because the training of the mind and awareness have been set in motion and conditioned to be in and maintain an awakened undistracted state.

Dimitrios X.
I think it would be better to go back to shorter sessions to retrain your concentration, then build up toward longer sessions again.
Another option could be analytical mediation. Many guided ones are available online, pick the ones that fit your needs. Then try to create your own ones. In religion, training on concentration meditation is often used as a startup for analytical mediation. Maybe you stayed for too long on concentration mediation only?