You are in a long term crunch here, the most important thing to do now, is to convert any province to your home culture, because this allows you to recruit at 100% of the value, if they are not in your subgroup then you can't recruit any troops. If they are in your subgroup you get 80%
Mercenary troops come in handy, but they will bleed your manpower reserves when they need to be reinforced.
trading grain? That will help the population grow and that helps with easing the next manpower crunch.
IN a real pinch I have adjusted the national idea to promote slaves to freedman because , freedman are the category that provides the troops. (slaves provide the money, and rich people provide the research.)
basically you have to lick you wounds and heal for a bit, but think of the long term goals for increasing your manpower base. In my current game i can pull in around 1500 men a month (Which is double, and i haven't completed the GC)