Getting more cautious with age is pretty normal, but it can get out of hand and paralyze you. I'm 56 and I have that in spades. My solution has been to figure out what I can do without locking up (but with a fair bit of fear), and keep doing it to desensitize myself to it. Then when it starts to get routine, push the limit a bit. Mentally you have to think of it as fun rather than as woosing out.
For your specific problem with boardslides, start with doing 50-50s then throw in a slight shiftie. Gradually increase the shiftie until you are boardsliding the second half of the box. etc.