Self motivation is a really hard thing to do so I totally understand where you are coming from. I recently finished my phd which involved lots of self motivation and there were so many days like you describe. Now I'm unemployed and still find it hard to self motivate.
Some things that helped were:
Getting out of the house and into a coffee shop first thing in the morning especially if you can do some work there. It helps to get things rolling.
Break your tasks up into little bits - the smallest possible bits - and have a couple of set things planned to do while you are out for coffee or when you get back.
Don't plan too much for any one day or you will get demoralised if you don't accomplish it, start small and reward yourself each time you achieve something.
If you find it hard to start working on something then commit to doing 15 min of work, often you will get into the swing of things and work for longer but if its not going well then you can stop and take a break after the 15 min are up.
Remember that some days you are not 100% and be kind to yourself and forgiving on those days.