I'm 18 i've struggled with depression since 11 i'm always tired and sad, i have so much to be happy about, why can't i feel it! what can i do?

Could be chemical: You are a good candidate for a chat with a psychiatrist. Especially if you have a family history of depression, this could be a chemical thing that's not your fault but may need a medication to resolve.

As above : U became depressed at age 11 , mood symptoms at an early age can be actually bad , which is a condition. That can be misdiagnosed resulting in receiving the wrong treatment for 10 years . U need to see a good psychiatrist. For evaluation and work up.