You should spend your last 10-15 mins by staying own half, playing defensive with counterattacks and keeping your team fresh by using all your substitutes. I like to use the first 2 at 55-65 mins and keep a last one for a case like that at nearly 80. You can also try to switch your formation a bit and use a more defensive approach. eg from a 4-4-2 to a defensive 4-4-1-1 or from 4-2-3-1 to a defensive 4-4-2 etc.
Don't give up ball possession to enhance your defense. Unless it's something really extreme, I would always prefer a midfielder for a defender. You should want to play defensively, keep possession and hit on counterattacks.