First of all, there really is no such thing as "too much masturbation." Now, can you bring yourself to climax during masturbation? If so, then there are most likely issues within your relationship that are preventing you from being satisfied. If not, then there may be a medical reason for your inability to climax, which I would recommend that you see your physician about...he will probably refer you to a Urologist.

You did not indicate your age. Of course, age can play a role in this possible dysfunction as well. Viagra may be the answer if this is the problem.

However, by the tone of your writing, I would speculate that you are having issues within your relationship, and your sex life is stagnant as a result. If your fiancee is ONLY willing to engage in the Missionary Position ("nothing more or less"), there is a DEFINITE spark missing from your sexual relationship at minimum.

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I wish all the ladies were pies on the shelf...and I was the baker because I'd eat them all myself.
I wish all the ladies were potholes in the road...and I was a dumptruck because I'd fill them with my load.