If I insert an tampon it gets stuck just past the opening. Can you give me some advice?

It’s possible that you push it in the wrong direction: against the wall of the vagina so it cannot go any further. So first explore your vagina with your index finger. Notice that your vagina is tilted towards your back. Becoming aware of the angle makes it easier to push the tampon in the right direction if you try again. It’s also possible you push in the right direction but that your vagina is too dry. So wet your vagina with saliva before you put the tampon in, by wetting your finger and inserting it. Repeat this a few times so you know it’s slippery enough. It might also help to use a tampon with a plastic applicator.