how come it makes it harder to go up the day of?
Deep breath ....
There are two different complex erection mechanisms going on in a full tumescense of the genital erectile tissue. Through manual or erotic stimulation, the penal sphincter relaxes to allow a greater influx of blood... but that blood can still just run back out. This, by itself, will only get you a chubby. The second process has to work too. That's the release of nitric oxide from epithelial/endothelial cells within the corpus cavernosum. That diffuses into the smooth muscle cells lining the arteries, relaxing the vascular/trabecular smooth muscle and allows them to become fully engorged with blood, and THAT compresses the veins that drain blood back out... keeping blood filing in faster than it can drain back out.
If the epithelium etc cells are damaged, or their chemistry (neurokinins, VIP, PHydromax, etc) is interfered with by healing byproducts ... they can't work right, so they can't expand right to trap the blood properly.
There's also a minimal trigger energy (parasympathetic innervation) for all of this happening the right way. If there's damage to the penile tissues, or impingement/damage to the lower spine, then the penis might not erect, ... or it might erect at first during foreplay or early sex but then give out and stop working.
Also, since this is a system, if you're not really emotionally 'into' the chick - or if mentally you're distracted or stressed out, this will also lower the innervation energy level.
Now, forget all of that and just get into it and have fun!