Definately seems like something wrong with your electrical system. I would check all connections to make sure they are clean and tight. Check charging system to make sure it's charging properly. Lastly keep the battery on a tender when not riding.

I keep all my batteries on a tender and they last 5+ years no problem. My riding mower battery is 6 yrs old and still kicking.

AGM batteries are tough but as in anything man made there are a few that do fail prematurely.

 

i had a battery last for 10 years and never used any kind of charger on it . it was a yuasa