What do I do?

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My husbands penis always goes soft when were doing it. It’s been going on for some time. He’s only 28 and it’s been going on for a few years. The doctors say he is fine but don’t offer solution. I get so upset but I don’t show it. It’s not every time but it’s most of the time. He beats him self up soo much because our sex is amazing. Has anyone experienced this ? Any advice..? His health is in great condition, he is a gym rat and army man. So I’m so confused and hurt.