Yes I hate EU4 too. But I'm addicted to it. So I just have to live with my dark passenger....
Now, solutions for you. First, on the opening screen set difficulty to EASY, disable "Lucky Nations", also iirc there may be a way to manipulate AI/player bonuses. Play on those settings until the challenge goes away.
Next: France.
1. Don't get into a land war with France. Even after the recent nerfs.
2. If you must attack them, make sure you have good allies that will join you (try France's natural enemy Burgundy whom you can ally from day 1) and make sure they have few allies that are willing to join them, or they're already in a war, preferably losing. Be opportunistic and cynical. Wait for the right moment.
3. If they attack you, you've done something wrong. Try having more allies, to deter France. Go over the diplo relations limit if necessary.
Next: call to arms from ally (in your case Navarre). If you're going to lose, or it looks iffy, don't join. Better to dishonor your alliance, take the diplomatic hit and move on. You also have cunning diplomatic options. For example, if you have a casus belli on the main enemy yourself (fabricating a claim on Poitou works a treat), consider declaring war on France directly, thus dragging your own allies into the fight as well as the "main" defender's allies, BEFORE you accept the call to arms from Navarre.
Next: Total War. Not even competition. I refer you to the gentlefolk who think you're a troll.
Next: manpower. Don't let it get close to zero as that will set up a dogpile by the AI. When you're down to say 50-60%, start consolidating your units like crazy so they don't drain manpower. Make up the numbers with mercenaries. If some mercenaries are good, more mercenaries are better. Take as many loans as necessary. Debt and inflation is far better than losing a war to France.
Combat: it's better to defend than attack. Particularly in the lovely mountains you have in or nearby Castile. Drain enemy manpower faster than yours.
This is a grand strategy game. To win you must have your grand strategy right. Get your alliances going, without the right alliances (and waiting for prospective enemies not to have the right alliances) you're probably screwed on both attack and defence.
HTH