Comment by world2vec
Whatever estimate you came up with: double it. You think it will take 3 weeks, conservatively? No, it's going to take a month and half. If somehow you deliver earlier then so be it but give yourself ample space to do things properly and avoid burnout.
Another thing, which I'm still learning to do, is saying "No" to new projects until you finish at least one of the WIPs.
There's a trap to be careful of here, because your estimate now is 1.5 months, so it's actually going to take 3 months.
The general rule is 80% of the work takes 80% of the time, and the remaining 20% takes 80% of the time. Try to get 80% of it done in 50% of the time.
So the goal should be to get development 80% done in 3 weeks and begin QA by then.