This may sound silly, but perhaps the best answer is to stop thinking of yourself as "behind." I don't mean to make that sound snarky or rude or anything, so I hope I'm making sense...
('Cause if you've done ANY scrapbooking, you're actually AHEAD of so many people who don't preserve their memories beyond iPhone photos or Facebook!)
I've found that NOT scrapping chronologically was easiest for me - so I never had to feel like I was "behind" in any way. The focus was then on what I *did* scrapbook, not on what I hadn't. And, it let me look forward to what I wanted to scrapbook, instead of feeling like I *had* to! So sure, I have a to-do list, or really, a wish list for scrapping. There are stories I want to make sure I tell in my books. But because I don't feel like I have to do it all in order, I can pick up the photos that are most interesting to me at the time, the ones that inspire me, or the ones that coordinate with the cool new paper I just picked up, or... whatever strikes my fancy. I like being flexible. So when I do a weekend crop (pretty much the only chance I get to scrapbook), I can start with my son's most recent birthday party pics, then switch to a layout about my husband and I when we were dating, move forward to a Disney vacation we took 2 years ago, then back to heritage photos of my grandparents... and not worry about what I'm missing in between.
The only thing I do pretty much chronologically is my Project Life type of daily-life album. And even for that, I work weekly, and my photos for the week don't have to go into the pockets in any strict order. Some weeks get extra inserts, sometimes a 2-page spread represents 2 or 3 weeks. So even within the chronological order of my pocket scrapbook, I find that flexibility helps a lot.
Don't let scrapbooking become a reason you feel guilty. Scrap what you want, when you can.
At least, that's what I find works best for me. If you really like keeping things lined up in order, then the anything-goes approach won't work for you. And that's okay - scrapbooking should be fun, and if you aren't comfortable with your approach, it won't be fun! In that case, I have heard from several scrappy friends that when they fall behind, they like best to start with the new (because the photos and stories are fresh), then gradually work backwards until they're back to where they left off. If you've got 8 years of unscrapped photos, that could take some time, so be patient with yourself, and have fun with it. You might want to keep your layouts featuring older photos simple - just your photos + words with minimal embellishment, so you can bang the pages out quicker.
You might even want to consider doing pocket-style scrapping for your older photos (there are lots of companies making pocket scrapbooking supplies nowadays), because that helps keep it simple. I have a friend who took all her old unscrapped vacation photos and did a big Project Life type of book with them - it's gorgeous, and gives a great overall view of all the traveling she did when she was younger, without her having to dedicate a huge amount of time to "catching up" on each vacation separately. Something like that might work for you...
I hope I've suggested something that helps...
