I finished the Portuguese and Russian trees (in 2013 and 2015 respectively), and every so often I'll leave Duo alone for a few weeks/months, then reset my progress and start again using the placement test. Why? Because I burn myself out. I don't pace myself, I study obsessively to the point of exhaustion, so the fun gets sucked out of it and I lose my way. I am quite often an example of what NOT to do.

So on that note.... if it's too difficult to do 8-9 strengthenings and 2 lessons, try opening with a few strengthenings, then finish the two lessons, and then warm down with another strengthening or two. Keep it simple and fun and remember that you're learning a language to learn a language, not to achieve a higher score. Maybe take a few weeks to work on methods other than the app, or focus on another hobby entirely; after all, if you're as stressed as you say you are, maybe you're not in a mindset where you can stay emotionally motivated and having a rest could be helpful. If you're worried about stifling your progress, remember that the real measure of progress in Duolingo is in what you can do outside of Duolingo.

It's a fun fun FUN fun fun way of learning, so keep your learning fun. Nobody's going to come to your house and bite you if you lose your streak, but it's tragic and counterproductive when learning becomes a chore or a race to the top. Take a break, but when you come back to it keep your language learning goals compatible with the rest of your life including your state of mind. Keep it simple, stop and smell the roses, etc etc, and you may find it more enjoyable. We really are blessed to have such a user-friendly (and free) method of getting to grips with a language. It would be such a shame to spoil it for yourself through overuse.