I have a major problem, I don't know if it is a bug or it's meant to happen, but whenever I try to go to Morthal, where Falion is (to cure me from vampirism (I have the filled Black Stone Gem)), everyone attacks me, including Falion, and he remains in his house during day instead being on the marches, where he usually is... I even tried sneaking into his house, and getting in while nobody was watching, and when I got in, Falion started attacking me! I can't get to him, pleeeease help!

You can't be in Stage 4 Vampirism to talk to him. At 1am, he makes his way to the unmarked summoning circle slightly north of Morthal. Follow him, you can probably continue the quest there.

Do you have dawnguard? cause if so then you probably have a bounty in morthal.

To fix that, go up to a guard and put your weapon away and he'll take you to jail.

If you don't have dawnguard then you need drink some blood and lower your vampire level before speaking to falion.

better stay away from garlic and garlic bread.

What they said. Make sure you've paid your bounty and don't be in the last stage of vampizm. The garlic is optional.

pay your bounty if you have it, also feed first (if you are not rping heavily) and go from stage 4 to 1