Alright, so I'm at the quest Alduin's bane. I got the elder scroll and I read it (no problems there), and the fight with Alduin himself is rather challenging!

Dragonrend works fine, but when I got Alduin down to around a very small amount of health, he immediatley flies up, quickly regenerates health, and the whole fight starts all over again. This time, I can't barely hurt him, because his health quickly regenerates, yet I can still use Dragonrend on him.

As well as this, I can't seem to use any other shouts on him when he is under the effects of dragonrend. My character "Speaks" the shout, but it just cancels itself out.

Is this a glitch, or is there a second phase which I don't know about? I turned on subtitles to find out what I need to do, but Paarthurnax just keeps telling me to attack him when he is downed. Even if I'm meant to use dragonrend when he is regenerating health, flames are everywhere, and my vision is blocked, so I don't know where he could be.

As for equipment I have, I've got a full set of Orcish armour (Exquisite), no enchantments, and I'm a Level 37 Breton (I really should of gone Mage, but the 50% magic resistance was tempting). 100% Perk Bonus to One-handed and heavy armour, and I've got a good amount of Health and Mana potions.

Well after beating him for the first time (get his health down to almost 0 I guess) he is supposed to take the cowards way out and flee to recharge his "batteries". Try loading a previous save maby before you red the scroll and see if it works then. I havent had this problem but when I red the scroll one time and watched the battle from the past Alduin never showed up. The three heroes just stod there staring so I loaded a save before I red the scroll and it worked normaly again.

I actually thought that I was doing so little damage to him, that if you don't beat him in a certain amount of time, he regenerates health and you have to do it all over again. I saw someone with a full set of Daedric armour manage to take him down in 4 hits, so I thought that Orcish wasn't recommended in the fight.

So I went back and did some sidequests to become stronger, rendering the "Read the elder scroll" part obsolute.

Is my Level 37 character... doomed to never be able to defeat Alduin? I really don't want to restart...

You are not ment to "win" the encounter at the throat of the world. He is supposed to flee at some point in the fight and you then have to follow him to his lair.

If you are refering to the second fight. Just keep at it I guess.

I know that there is a second (possibly third?) fight with him in the future, but when he begins to regenerate health, I can barely damage him because whatever damage I inflict he regenerates it. And even if I get his health down to the point where he is "Supposed to flee", the fight continues on, but this time I can barely damage him.

Is this a glitch or has one of the mods I've downloaded done this?

It might be a glitch yes and it might be the mods, plz tell us what kind of mods you are using and it will be easier to help you... hopefully. xD

if you attack him before he finishes speaking to you, he can become invinsible.

Alright, so I have the following mods: (Do I need to provide links? I'll add them in if its necessary)

Non-essential Childs (NO I do not kill children in Skyrim, this is mostly for a "Realism" thing where I defend children from Dragon attacks)
Legendary Lore - Dragons (This boosts the difficulty when fighting a dragon, it also gives them shouts like Unrelenting force, Disarm and Ice Form, this MAY be the reason for this glitch)
Killable Essential NPCs (Blargh Thieves Guild)
Dragon souls for perks
Reduced Distance NPC Greetings (Do you get to the cloud district very often? Ah, what am I saying? Of course you don't.)
I also had certain mods that modified smithing and allowed you to craft certain armours you couldn't, but I removed those.

Next time I play Skyrim I'll disable all my mods and see if things go normally.

I waxed his ass at around level 18, but I think you're not nimble enough. I don't think there are any glitches.Your armour is probably of little use here but I don't know , I'm a mage and used entirely ranged spells and a Flame Atronach. The minute he hits the floor you must be on him. He is invincible when flying so hide, peek out and dragonrend him, attack him when he's on the floor. When he flies off hide and repeat. Drop a level of difficulty is you have to. Keep moving, move fast and use a good sword and get behind him.

Best fight in the game for me.

Try uncheck the "Legendary Lore" mod and see if it works without it running. Also read Aphaea's post and see if you've done what she/he said.

Alright, so I unchecked all my mods, and I managed to beat him! Now I feel stupid...

Glad you got it sorted.

Heres your http://www.uesp.net/wiki/General:Fishy_Stick