There are some great answers here, truly. I am only going to add to it from my own perspective.
I have had a side business with a partner since 2006. In that business we might have turned over £50k in all that time. We aimed for it to be a replacement for our day jobs. The problem is the exact problem you have.
About two months ago I suddenly lost the ability to use my legs thanks to an abscess compressing my spine which was removed and have undergone rehabilitation. I'm still on crutches, and hope to be able to make a full recovery within a couple of months. However I spent 2 months thinking about this a lot.
What's interesting is that I could have spent 2 months working on side projects - I was off work, an inpatient in a hospital with WiFi and my laptop. Instead, I decided to take stock.
I then realised the key was the rehabilitation itself was giving me the answers, and I concluded:
- Whilst having big goals is awesome, you need to focus on the here and now and doing the work you can do right now
- Small and often is better than big commitments
- Celebrate small victories. Got 5% more done than you did last week? Well done you - keep that up, and in a year you'll be doing 12 times what you did last week.
- Keep a log of what you have done once you've done it, with dates. Once a month go back and reflect, and congratulate yourself for the progress you have made. Seriously, this helped me so much - it's a bit like not wanting to break the chain.
- Give yourself time to rest. Your body really, really needs it.
- Things will rarely work out quite as well as you hope, or as badly as you fear. Just do the work, and see where it takes you.
- Motivational videos got me out of slumps a few times. These both helped me at different points:
https://www.youtube.com/watch?v=hbkZrOU1Zag (technically a Nike advert, but I have the audio of this on my phone and listen to it a few times a month)
https://www.youtube.com/watch?v=flKP4Te8L94
All of this got me through rehab, and I'm starting to see it drip into other parts of my life including work and side projects.
Don't be so hard on yourself, but remember the best laid plans are worth nothing: go get a pomodoro timer and do 25 minutes on that project right now. Then do another tomorrow. And then perhaps another straight after. Have a days rest between if you want.
If you find yourself doing two of those pomodoros every other day, you're going to look back after a few weeks and realise you've actually got quite a lot done.
Good luck.