Staying in the moment

I think thats what the pros refer to it as.
Yesterday, I got off to a great start - three under through five on Lakeside.  Then came the rain delay and I started thinking about possibly breaking 70 for the first time.   That was the end of that.  I preceded to double bogey the first hole after the rain delay.   I ended up shooting 73 but it should have been better.
Once I started thinking about shooting a low number, I started playing like I was hanging on for dear life.   Everything became tentative.
The thing is, this has happened before and its a mental barrier I need to find a way of getting over.   There's a lot of cliches out there ("stay in the moment", "one shot at a time" etc).   Not sure how to address this one!!
On a more positive note, the driver and 3W are working very well and putting is markedly improved.