First and Foremost, you should stop playing as a count, it is no longer as straightforward as it used to be. By playing as a weak starting character, you are intentionally handicapping yourself for a game you were completely unprepared for. Try playing as a french duke to begin with. That means that you will have the power of your liege for defensive wars, and can declare war on people much weaker than you, ie rebellious counts and weaker dukes. You should start out only going to war with people who you outnumber by 50 percent, minimum, since you have to deal with seiging and defended bonuses.
You should try not to rely on warfare. Making the right marriage matches and waiting up to an hour for them to come to fruition is part of the game, (playing the long game is part of all paradox games really). In fact, Marriage is actually a much stronger mechanic for taking territory than combat. Not only that, but marriage is the most powerful mechanic in combat as well, because it allows you to call allies with vastly larger militaries than you can command early in the game. An alliance with the king of NearbyNation will let you call in thousands or tens of thousands of additional troops.
Still, the overall point is to stop playing as weak characters. Counts and the occasional 1 province duke in ireland are not advisable start locations for someone unfamiliar with paradox games, because like real life, paradox combat is very swingy. A 20 percent difference in starting numbers can be a 200 percent difference in casualties. That means when starting out, make sure you absolutely have a substantial numerical advantage against whoever you declare war on.
As for your example of losing a battle where you had superior numbers, you probably made a mistake with regards to combat morale, but its difficult to know without screenshots. Even if they had a vastly better general you should not lose a 1000 to 2500 fight, unless you had virtually no morale (probably from not paying the men).
Basic Concepts to Practice:
Revoking a county title as a Duke. This usually starts a war of rebellion that you can crush.
Invading an independent county as a Duke. This includes ones rebelling from their liege that have become independent temporarily
Marrying daughters into a powerful alliance (nearby, but not where you want to expand to)
Marrying sons to inherit land
Keeping your most important vassals and advisors (particularly the spymaster) happy.
Sounds like you're boned. There's a few things you can do.
Current situation and options
First of all, find out where is his new army coming from. Are you allowing his retreating forces to regroup, or is he hiring mercs? Find out how many armies he can still raise. You could ignore the holdings for now, and destroy his routing troups, and then crush any newly raised forces before he can doomstack.
Be prepared for a long struggle – basically, you need to use your main army to pounce on any small (or morale-deficient) enemy armies, while a minimal portion of your army liberates territory one county of the time.
Keep an eye on his cashflow, though. If he's loaded, he will probably hire mercs sooner or later, so you need to peace him out before that.
Alternative solutions
If you're lucky, you can use your chancellor to sow discontent, and spark a rebellion. This will only work, if one of his vassals are already discontent.
Assassinate that mother. It'll cost you, and it's far from guaranteed. It might fail, and he will send a counter-assassin after you, which might not be such a bad thing (more on that later).
Also check if you can have the pope excommunicate him. If he has any enemies without a CB, they might use this opportunity to make his life miserable. Likely this isn't an option for you, but it never hurts to check.
Your ruler and your relations
I don't understand why you can't work from your situation as a child – you should have the same options as a normal ruler, only with severly gimped stats and no children. If you have a cute sister, you should be able to marry her off for an alliance. Invite people to your court and force-marry them, if nescesarry.
If you get the "wrong government type" alert, that means you are ruling something you shouldn't. Go though all of your titles and county screens again. Perhaps you are the de facto ruler of a title, that does not actually exist for some reason? Unless this title gives you a load of levies, get it fixed (by creating it or giving it away).
Do you have a dynastic heir? Perhaps an adult uncle or something? You know, maybe he should be ruling instead of you. Maybe you should fall down a well. Here's some ideas on how to make that happen. This will give you more options as an adult ruler, and might even simply end the war inconcluseively due to a change of management!
Conclusion
Lastly, be prepared to accept losses. If you can have him agree to any peace terms that doesn't involve you losing the game, consider it strongly. You will have CB on any lost territory, even if you become a vassal. It sucks, but it's not the end of the world, it's just a new development to your game.
Best Answer
Just chain assassinate the bastard.. put your spy master around his army and cross your finger.. If the claimant is dead then the war is over without you could rise your country levies.