How to stop it from sliding back and tilting upwards I can’t tighten it enough, no force stops it, I even tried double sided tape to add friction.

In my situation I had to really tighten it down and it seemed to help on my last few rides

This frustration is why i have switched to 2-bolt seatposts

This is why bmx got rid of clamps completely

I once filed shallow grooves on the top of the post to give it more grip, seem to recall it worked

Take the clamp and saddle and seat completely off the post. Examine the surfaces that the clamp ….clamps onto.

This is often knurled or machined so they can grip together. If they have been rubbed smooth then we can (carefully now, we want to keep our fingers) cut with stanley knife or a junior (or bigger if careful) hacksaw some ridges back in.

If this works, dont be tempted to ride around with a loose seat clamp as they can get damaged easy.

Sometimes the clamp is bent and a vice can come in handy, sometimes the bolts are stripped.

Its hard to get spare clamps as every manufacturer seems to use their own system so unless you have access to a good used spares/recycler one might have to buy a whole new post.

Pointless usually jamming bits of rubber or plastic in, although a metallic shim might help in some limited cases.

Basically your clamp needs repair from being damaged and ridden around whilst loose.

Just get a new seatpost. If it won’t tighten, the mating surfaces are compromised.