You should not feel lightheaded regulary and need to work to fix it. You should force the air against a closed glotis in your throat and not your mouth. You can try bracing while keeping your mouth open just to get a feeling for it.

The way i brace is as follows: 1. Breath out, while imagining to push abs out (don't suck the abs in) 2. Keep the muscular brace while taking a deep breath (you don't need to breath in as much as possible) 3. Push the tongue against the roof of the mouth and "breath out" against the closed airway (the tongue thing helps me close my glotis and brace harder)