I was building a big house, when I decided to take a break and make an end portal. I went through, beat the dragon, but when I stepped through the return portal, I was in an entirely different spawn! I tried using the compass, but it reset my spawn point to where the portal let me out! What do I do?
Are you sure you’re not confusing a player spawn point with world spawn? The end portal puts you at world spawn and compasses point to world spawn. Your bed spawn is completely separate.
Possibly Find Bed
You can use maps to find your bed, or try to look for disturbances to find where you set up camp. Find some sugar cane and make a map, if you make it big enough you might just find a discovered area dotted with buildings - your goal!
This is why you should usually make some sort of path leading to your base from world spawn, this can be easily be done by making a trail of torches and starting at your town and walking to spawn, using a compass to find it.
Killing yourself will make you spawn at your bed (assuming you have a bed, obviously, if not you just spawn back at world spawn). You can kill yourself and use a compass to maybe find your items later, although most of your XP will be gone. Obviously this should be the last resort but is effective. The best way to kill yourself in this situation is probably repeated fall damage, you might want to remove your armor.