your goal really is achievable, so take heart
Missing classes early on does make it hard to catch up, because you need to have a handle on the basics. It sounds like you haven't yet been able to regain the ground you lost when your nan died, and may not even have had enough time to properly mourn her.
Have you spoken to anyone at school about this? There may be some help they can offer you - general support, perhaps, as well as ideas on how to catch up. It may be better, depending on how far behind you feel you are, to look at repeating a term, so that you can regain some confidence a well as making up what you missed.
I haven't been in the same position as you, but I spent my last years at high school, and first year out, weighed down by the idea that if I screwed up my whole life would be affected. It made me so anxious I couldn't get any perspective. What I know now is that, while it may be easier to get it all right the first time around, there are second chances, and opportunities to make up for problems earlier.
I'm so not expressing what I mean well! How's this, to better illustrate what I mean - I dropped out of uni first time around, and for years felt like an academic failure, even after finishing off a couple of Masters degrees. Then I spoke with my uni superviser about it - [I]she[/] dropped out of high school, had a couple of daughters, bummed around for a while, is younger than me, and is now a lecturer.
In other words, if you feel like you need to take some time, get a grip on your life, it'll be okay.
Hope this has helped. Good luck!