Hey Jean-Francois, welcome to the DhO,
Given the length of time you've been meditating for, and based on the assumption that you don't have a lot of meditation experience prior to this since you haven't mentioned your previous practice, it's likely that you got into 1st ñana, Knowledge of Mind & Body. It's not uncommon for this to be quite a big deal if you've never experienced it before so your descriptions make sense, also the following stages of insight, namely Knowledge of Cause & Effect and Knowledge of the Three Characteristics, can be quite unpleasant, particularly the 3rd ñana.
Now I'm having trouble getting that focused again, I find myself having little motivation and I have weird mood swings from depression (mainly when thinking about world news/problems, to easily accepting things that happen to me (missing the bus, breaking my shoes)
Don't strive for what's gone before, it's a waste of time because progress can only happen
now.
Look at what it is about your experience that's different when you're feeling "depressed" and when you're "easily accepting things".
Who or what experiences the feelings of being "depressed" or "easily accepting"?
Also while meditating, sometimes I just stop focusing and let sensations arise like they want...
Good, now notice how they arise, where they arise from and where they go to. See if you can stay with the entire sensation, from it's arising to it's passing away, watch it, observe it, look at how each and every sensation that arises is fundamentally the same.
...sometimes I focus on different parts of my body and all the sensations that make it up for like 5 seconds at a time, sometimes I find myself wanting to solidify pleasant feelings I get from vipassana meditation or when I get close (or closer as I haven't reached it yet since I've been spending more time on vipassana) to the 1st samantha jana, I find myself switching to feeling all the sensations that make up this state.
It may be that you're more naturally inclined towards insight than concentraion, it's worth knowing that insight practice will take you through the same jhanic strata as concentration will but not in the same stable, solid way. If you notice yourself wanting to solidify things, just turn the attention back towards those sensations and see their impermanence in the way they all just arise and pass, see how any sensation cannot be that which is observing it and so can't be a self, and how even seemingly pleasant sensations contain an inherent push/pull duality manifesting as mental tension. This is observing the Three Characteristics in real-time and what leads to insight.
So I was just wondering if stumbling across the first stage on the map presented in the book can cause these things or it might be something else? (Even completely unrelated to meditation, like my small but active efforts to train in morality)
Yes, what you're saying is accurate but it's worth being aware of the possibility of "scripting" yourself into experiencing things in a certain way. The Buddha taught morality as the first training so perhaps your efforts are paying off!
Practice well and let us know how you get on.
T