Playing standard tuning slide guitar is difficult. Because you’re not in an open tuning the overtones sound terrible if you’re not damping properly.
In open tuning you can have overtones ringing and they’re in tune because the guitar is tuned to a chord.
The important thing is not to get discouraged. It sounds like crap in the beginning for everyone. When someone has been playing slide/bottleneck in standard tuning for years it looks and sounds easy. It’s not.
I started playing slide/bottleneck in open tunings and it wasn’t until I was at a friends gig and heard him playing in the key of G in standard tuning that I really got interested in standard tuning slide guitar playing.
One of the many benefits of playing slide/bottleneck in standard tuning is you don’t have to re-tune the guitar. You can just improvise on the fly. It takes some practice but once you get the damping and intonation down the fretboard is wide open.
Once you get proficient at playing standard tuning slide guitar, it’s easy to make people think you’re actually open tuned. Usually the best keys to start learning in are E, A, and the key of G, all in standard tuning.
There are a lot of players teaching this basic style of guitar playing on youtube. I just learned the hard way by figuring most of it out myself along the way.
It is tough, but stick with it and it will come. Then you’ll spend the rest of your life refining it.