I would go back 5 years and say: Everyone around you is taking advantage of you, stop being so nice all the time! It's great to have and help friends, but put yourself first sometimes. You don't need to shoulder all of their problems.
I dunno, Grandfather paradox means it really doesn't matter what I say unless you follow the Many Worlds Interpretation or multiverse theory in which case I'm not changing my circumstance but essentially putting a clone of myself on a new pathway. But it might be fun to at least see what he does differently. So I'd just tell him to keep working out, don't ever forget to tell your loved ones how much you care, and to not be so afraid of doing things you want and that it's okay to be a bit selfish sometimes.
I'd go back in time many years back and tell myself this; "neet-one, on may 9th of 2016 there will be a super awesome mega incredible thread on maiotaku.http://www.maiotaku.com/mytopics/561/mytopics/29619 It's very important you post in this topic and tell your past self to do the same thing."
I'd tell my self to move a way from the place you don't fit in as keep trying to work out how to fit in will lead to no-where and to believe you have strength to go it a lone.
For me I'd tell my middle school self that I don't have to be alone.
Also..."Knock that fake bitch out".
and
STAY AWAY FROM THAT BIG ASS DOG!!
I would go back 5 years and say: Everyone around you is taking advantage of you, stop being so nice all the time! It's great to have and help friends, but put yourself first sometimes. You don't need to shoulder all of their problems.
(I don't think my past self would listen though)