If your therapist is a real therapist, then he will accept your condition that masturbation is unacceptable to you - at least as much as your rabbi does. After all, it was your rabbi who first told you that guilt feelings are unproductive and not the proper way to respond to your habit, as the problem you have is quite common. Your wrote:
My Rabbi always tells me that it is natural to have these urges and I am better than everyone else for just trying to stop. Is it also natural to not be able to stop.
So. A therapist (unless he is a religious-based advisor - like your Rav is) is bound to be non-judgemental, he would work with a homosexual couple as diligently to work out what they describe as "marriage trouble", as he would have to with a normal real married couple. By the same token, even though he may in fact believe that porn and masturbation ae good ways to deal with stress of life and growing up, still - if it is
your personal goal to live free of it,
that is the goal of the therapy.
But the trick is that you must make this clear with the shrink at the outset. No question. You do not accept having masturbation a normal part of your life, period. If this is what you believe and your goal - say it to him.
And at the same time, ask him point blank - put an end to the fuzzy mushiness and (comfortable) confusion - "Can
you honestly work with me to achieve
my goals - or do you feel that due to your personal beliefs, you will try to sway me toward accepting masturbation as an OK outlet for myself?" Ask him point blank. If you cannot bring yourself to do that, then explain this to your Rov and ask him to do it for you with the shrink. And if you or the Rov get the impression that this guy is nor safe or nor sincere, then look for someone else, quick.
It is simply not fair to you - or to this therapist - to just leave out a major ikkar of your problem because you simply cannot trust him to be honest with you. It smacks of the same old secrecy that we all use to go off hide with our porn and masturbate.
You will get better the open way, not the hidden way.
Hatzlocha!!