First, don't panic. You're tired and I'm pretty sure it looks worse to you than it is- most jobs do when you have just finished, you see anything that's not perfect as much worse than you will in a week or two.
It looks to me, and I'm guessing here just on the basis of the photos, that the sanding maybe was a bit uneven- like it was done with a belt sander and has dug in in some places, and has sanded further into the wood in some areas than others- possibly where planks weren't completely flat, as old ones often aren't. Sanding a floor is a horrible job, and very hard to get right. If I had to use a belt sander, I'd be very careful not to dig in, and I'd finish off with an orbital or similar to be sure it was really smooth and flat, and sand into the wood until it was all smooth, one colour and one texture and all the previous surface finishes had completely gone. If some areas are given their final sanding with coarse grit papers, they will tand to stain differently to ones that have been given their final sanding with finer grit ones, so work from coarse to fine as you go.
But as we are where we are, okay, let's just break the problem down a bit. You'll find the floor gets more even with time if light gets on it, as it will darken all over and get a bit 'warmer' in colour, and that will reduce the difference between the darkr and lighter areas. You may well find it ends up looking fine, and you need to do nothing. If that doesn't work,
- were the floorboards originally stained or varnished at all? If so, it may be that you didn't get all of the stain or previous finish off, in which case resanding is the only option that will get it totally even in colour. If you can't face that now, which is understandable, I'd live with it for a bit- the surface will wear down and get somewhat more even in appearance over time, and if you still think it's patchy, it will be easier to sand later when you feel up to it.
- If it was all sanded and completely even, flat, and light in colour before you painted, did you stain it first before varnishing? If so, it may be that you didn't get it on evenly. Staining is very difficult, you can't touch any area twice or it will get darker than the rest. The remedy, sadly, is the same as the above.
3.If it was well sanded and the colour is in the varnish, then like with a stain, it may be that you didn't get the first coat on thinly
and evenly, which could have made it patchy. In that case, I'd again leave it and 'walk it down' but when you feel ready, you could give it a light sand and try doing another coat, that may even it up, provided you don't mind it getting generally darker.
(Personally, I oil real wood floors, but that doesn't suit everyone.)
Try not to make decisions about it when you're tired, and good luck.