I'm just like you in getting attached to animals. 
Before you should add more fish you should try and cycle your tank. Cycling is establishing a cycle of bacteria that turns the toxic ammonia (which comes from the fishy poop lol) into nitrate which is much less toxic. It's not too tricky but it takes a while (about a month :$). You can do it with your betta in there! Here is a good thread on it: http://www.bettafish.com/showthread.php?t=47838
Your tank isn't large enough for any other algae eaters other than a nerite snail or otos.
Otos are quite fragile, so they usually like to be in a well established tank (up and running for over 6 months), so they probably won't be your best bet.
I love nerites.
They eat algae like a champ, are small and don't breed in fresh water.
Apple snails get big and are messy. They poop TONS! lol They aren't very good algae eaters.
Another option is to clean of the algae yourself. I'm not sure about live plants - but when I get algae on my fake plants and caves I just take them out of the tank and rinse them under hot water. With a little scrubbing the algae comes right off. When it gets on the walls of the tank I use my gravel siphon to suck it up.
How long do you have the light on in the tank during the day? I'm guilty of having it on too long.
In my experience, if I leave it on for 12 hours or more in one day algae will appear.
I hope that didn't sound too intimidating. :/ Fish-keeping is a blast and it's really rewarding.
Good Luck!
Before you should add more fish you should try and cycle your tank. Cycling is establishing a cycle of bacteria that turns the toxic ammonia (which comes from the fishy poop lol) into nitrate which is much less toxic. It's not too tricky but it takes a while (about a month :$). You can do it with your betta in there! Here is a good thread on it: http://www.bettafish.com/showthread.php?t=47838
Your tank isn't large enough for any other algae eaters other than a nerite snail or otos.
Otos are quite fragile, so they usually like to be in a well established tank (up and running for over 6 months), so they probably won't be your best bet.
I love nerites.
Apple snails get big and are messy. They poop TONS! lol They aren't very good algae eaters.
Another option is to clean of the algae yourself. I'm not sure about live plants - but when I get algae on my fake plants and caves I just take them out of the tank and rinse them under hot water. With a little scrubbing the algae comes right off. When it gets on the walls of the tank I use my gravel siphon to suck it up.
How long do you have the light on in the tank during the day? I'm guilty of having it on too long.
I hope that didn't sound too intimidating. :/ Fish-keeping is a blast and it's really rewarding.
Good Luck!