There are many times in life when people feel their confidence desert them, and it can happen in any relationship between two people. One person might suddenly experience an event that makes them question everything around them, and their feeling could very well include the person closest to them. Rather than face it head on, they nibble around the edges to see what they can discover. This can ruin a long term relationship in a short time, so it is best to confront it.
People are often afraid to confront their biggest fears, so they find ways to avoid them. If a person is afraid of closed spaces, they will live in a home with large areas that are open, and their vehicle might be much bigger than what they require. If the phobia is something that was acquired during childhood, they might be able to figure out what it was and face it, but many of them will simply make accommodations in their life to keep it from being triggers. It is the same way with relationships.
When a person suddenly loses confidence in their partner or their self, they need to address the issue immediately. Losing confidence in a partner will lead to accusations and arguments between them, and the relationship could end quickly. They will retain their feeling of lack of confidence, and it can color their ability to be with others for the rest of their life. Facing it may give them a chance to recover without losing a cherished partner from their life.
If their feelings of confidence are solely their own and do not extend to their partner, they still need to be examined. A partner can help them get through the process while feeling safe and protected. It can strengthen the bonds between the pair and help them continue a relationship that will last a lifetime.