Im wondering once I hit level 20 and use sell abilites, when I drop to level 19, I get all 19 credits back correct? and after taht I can still level up to 20 and re earn my last credit, is that correct? I don't want to permanently lose anything

While i Have not respeced from lvl 20, I have respeced several times from lower levels and you do get all of your credits back from the previous level down. So you would get 19 credits and then when you hit lvl 20 you would get your last credit.