I understand you!
At first i found alcohol to be a very good friend on that... It worked for a few time, because after some time, it just doesn't work anymore.
What i do now is to focus on the game and the game only. Just looking at the table and balls positions, how to get them when my turn at the table arrives. Planning the run out, planning how to get a cluster resolved, that puts your mind on the correct side.
When going down to play, if nerves are around, just get up, go round the table, drink a little water and repeat the pre shot routine again. It works a lot.
Now, even with the mentioned before, what the opponent does, affects me: if a player plays to fast, i also play fast and i make a lot of errors with that. If a player comes very dominant on is presence (think about dart players, with all that flair and show), if a opponent comes to the table on that fashion i get anxious and feel inferior, losing focus with that. If a player at the warm up does some spectacular play i get anxious too (i do a lot of that purposely to weaker opponents, it works with some people, i am one of them lol).
As you can read, a lot of the game hides on our mentally, we just need to find how to resolve those issues in our favor. Not a easy task, i can assure you, but it can be done.
There are a few books that point to methods to overcome nerves and push out the best qualities, however, i have read 2 or 3 and couldn't understand them properly. Maybe if you read something about it it can help you out.
Professional choices: hire a sport psychologist, hire a nutritionist and hire a coach, that will improve your game fast and big.