I have found that my body goes through different periods; sometimes only a vibrator will get me off; at other times, a vibrator is just too much and only manual stimulation will work. Sometimes I have trouble coming no matter what I use; and some of THOSE times, I find that switching to something else in my head will help. I can't come when I masturbate unless I am fantasizing, so at times changing what fantasy I am using does the trick. I also find that stimulating more than one erogenous zone at time helps me to get over that edge--such as G-spot and clitoral stimulation simultaneously.

I know how frustrating it can be. Right now I'm going through the same thing and have been for quite a few months. I don't know why I stop; once in a while I'll force myself to continue, but usually I stop and then try to start again. Usually, though, I will eventually come if I continue to try to light that "match." I guess that's where having a partner would be helpful; a partner would probably force us past that point where we, in solo sex, would stop.

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Communication is the key.