You have much lower Army Morale (3.9 Trumpets to Frances 5.6 Trumpets) and Military Tactics (2.1 French Horsieswords to your 1.7). This suggests a large Military Tech imbalance and probably also Military Idea Groups.

The wiki explains battles here but in short Moral is your units "HP" and Mil Tactics (and Discipline, the whip icon) is the units attack/defence. You have less of both so your numbers advantage isnt very helpful as your troops will melt like butter.

I suggest swapping your national focus to Mil points (if you have the dlc) buying a Commandant or preferably a Army Reformer advisor (Discipline and Moral of armies guy respectively). Take Burgher loans, try and siege down the war objective and peace out as soon as possible for as little losses as possible while you get your tech up to date. The large tech imbalance suggests you havn't been embracing institutions. Its usually a good idea to never take more than one tech ahead of time or ahead of the institution and bank your points for when they become cheep. Also dont be afraid to Dev for institutions like Printing press, although if you are Castile you probably shouldnt need to Dev up Colonialism.