This is where data would be helpful.
Some folks don't believe in all the jargon, but if I asked someone "what's your TSB been the last two months" and they said "negative 20 for most of it"........ahhh, there you go. If someone has a hugely negative TSB for seemingly forever, they either get injured or feel like dog crap and can't perform..........or if they feel good their zones are based on a bogus test result. So jargon can be helpful.
Or, if you could say you ramped it straight up to 12 hours a week after being off a month.......there you go.
You have to ramp training load. I say load also, because time is subjective. I could do three 4 hour group rides per week and never hardly break a sweat because they're easy cruises.
Shoot, even just look at the bar graph by week on Strava for an idea of how you ramp your load.
I won't get into the medical stuff like lightheaded stuff or otherwise. I'll leave that to the pro. I get that rarely but usually is as simple as a piece of fruit, tumbler of water, and a short walk to get the balance back.
This sounds like maybe a good scenario for a coach to assist you with goal
#1 being to get in control.
You'll get it sorted! You can do it.