Iam No pro like alot of you are, But I like to think i can do a few things, how else do you learn. I put up a new back door, and it was the standard 36x 81, I got it in but when I close it like something is causing it to close hard, I can get it to close and lock, but you have to push very hard, also as your closing its like something is draging on the bottom, Have ran my hand across the bottom of Door, all clear. Its level, I need so help please....
You may need to start over. But check if the threshold is adjustable. If yes, is it all the way down??
To start over, check that floor is level. It is helpful if the wall is plumb, but not a necessity. Install the door in the opening, and roughly center it. The threshold must be level, shim as needed. Now plumb the hinge side, both directions. Shims at each hinge, with 3" screws into the house framing(usually supplied with a new door). Check the gap around door. It should be even and the door should close flat against the jamb weatherstrip all the way up. If you have that, secure the latch side jamb ater shimming. The jamb at the latch should be secured with 3" screws into the house framing.
All good info, something else that often happens is the hindge jamb can get twisted if it's not shimmed with two shims, one on each side, so it applys equal pressure.
If it gets twisted it can throw off the exposuer enough to cause the door to drag at the top or bottom.