If your Dom (or Domme) is a good man (or woman) (s)he will want to work through this issue with you. Patience is one of the Dom/me's key tools, and They will want to use it to help better your life as an individual as well as Y/your lives as a D/s pair, if They are in the "good egg" category. Not all Dom/mes, of course, fall into this category, so be wary when you're choosing a Master or Mistress that the lines of communication are open, that They seem receptive to your ideas and limitations, and that They express concern for your wellbeing and physical and psychological health (and the maintenance of that health).

Not all of BDSM is about cumming. If it isn't a huge issue for you, find a Dom/me who also isn't interested in cum restriction, forced orgasm, etc. There are plenty of other kinks out there to experiment with. For example, My Pet has a physical disability, so there are a lot of things W/we can't do. However, she is fascinated by (and fearful of) My complete control of her, and it turns her on very much if I use a particular tone of voice, even without touching her. She might not cum, but the rush of endorphins and adrenaline is still there.

Don't let the issue stop you. There are plenty of Dom/mes out there who would be willing to help you with it, or even who wouldn't care that you can't cum and would be happy to get involved with you in other ways. Just remember that no matter who you choose, your Dom/me should follow the rules of safe, sane, and consensual and should respect you. It is his/her duty to do so as Dom/me, as well as to protect you and look after you. Your role as sub should simply be to serve your Master or Mistress as (s)he directs and to do so without question. If you feel like that role would make you happy, maybe cumming is not really a necessity in your D/s relationship.

Hope that helped!! Good luck.