You might have been able to hit heads a long time ago, or you rarely hit heads. If you are a good sniper, get out of here this guide isn't for you. Good job though keep on that.
I too, was s#it. I took a few months off sniper, and got even more s#it.
So, here is what I did that actually improved my sniping skills.

So you may or may not have realised that TF2 has a practice mode (Find a game>Training>Tutorial/Offline Practice). The tutorial is a bit lacking, providing only help and information for the classes that may be a bit hard to understand to new players (for some reason, no medic).

Click Offline Practice.
Select King of the hill.
Select Nucleus, since it has huge sightlines and can help snipers.
Use 23 bots, although less or more doesnt make much of a difference.

Select Sniper.
Make sure you have the stock loadout, will help.

You'll soon find that you're somehow hitting more heads (I don't know how, but yeah it works.)

Only cap the point when it's very clear, otherwise just stay back and let your AI teammates do it.
It does not matter if you are winning or losing, as long as you are hitting heads.

Repeat this process until your scoreboard (tab key) says that you have hit 100 headshots.
Once you hit 100 Headshots, go onto ctf_2fort and do a little bit of calm sniping from the balconies and doors. If there is a good sniper on the other team, just play another class until you're sure that sniper isnt gonna bother you.

It worked for me.
I started hitting shots I would've never hit.
I started consistently getting at least 10 headshots a match.
If you start to lose your sniper touch again, repeat the process with a goal you set yourself, 50 heads, 150, 200, whatever. As long as you reach that goal, you will get better.

Happy sniping!

Thumbnail taken from LazyPurple's "How it feels to play sniper" video.