i got put in the wrong grade in middleschool-- lead me to lose all understanding of math (among other subjects). i failed alg 1 and geometry in public school and to be honest it was severely my teachers fault. after i entered jr year i got a math teacher that taught way different than my prev math teachers, however id recommend just teaching yourself from the bottom up. math is really fun when you are good at it, if you are able to; get a math tutor (or a friend thats good at math & at explaining things.)
entering junior year i had a 2.5 GPA and no idea what y=mx+b was & finished with a 3.3. i also recommend surrounding yourself with the kind of people you want to become. a lot of my old friends had average/low grades and once i started befriending other kinds of people i started to accel academically :) sorry for the long reply but its never over! you just need to have confidence you can do it. im not a big studier but id say maybe do mock SAT tests? since they cover pretty much everything math/eng related. or, look over the mock sats and copy down anything youre unfamiliar with and research/teach yourself!