How can I reduce vaginal wetness during intercourse? Apparently this makes my man go soft. Am I the problem?

Vaginal wetness lube. This is an interesting problem reported by a few of my patients for which there are few options. On the more radical side, I have heard of gynecologists performing cervical surgery to reduce the secretions. There are no widely available vaginal drying agents although a various agents have been used off label. In some instances vaginoplasty has been used to reduce the caliber & increase friction.
It should not. That is normal and is a sign that you are aroused and normal for you; possibly he should take more time with heavy petting and stimulating you in areas that you would like and that he would like. Normally lubrication is a good thing.
I agree. And i wouldn't recommend trying to reduce the natural lubrication that is supposed to occur during foreplay and intercourse. If the wetness is more watery, you should be evaluated by a urogynecologist or urologist.
Man losing erection. It sounds like your partner is having difficulty maintaining his erection. There can be a variety of causes for this and he should have a medical evaluation.