Hi ~kay~ - a few questions - you mention your master's age- and imply quite an age gap - does the age difference intimidate you, or turn you off? If it does, then that may be the start of the issue for you.
If you are excited about the age difference (or at least aren't upset by it) then the issue may lie elsewhere...for example - does your master stimulate you in a sexual sense during play? or does he focus on the pain side? It may be that you need sexual (sensual) stimulation as part of the pain play- at least initially.
Anxiety can also act as a damper for you - you indicate some sense of anxiety as well. If anxiety is the source of the issue, you'll need to address the anxiety to be able to relax enough to cum....
The fact your master is implying that your inablility to cum may relate to lack of a relationship for some time is suspect. That isn't usually a problem - in fact, a period of celibacy tends to have just the opposite reaction (in both women and men).
Euryleia has some valid points about seeking medical advice - there are medical conditions that can cause female dysfunction - it may be just your anxiety is causing the issue - and if that IS the case - both you and your dom may be putting additional pressure on you thus compounding the issue.
Best of luck
W