Reservations about freelance work

I haven't written here much lately because I've been heads down, blogging on the site that I was recruited to write for.  I have virtually doubled my income weekly, which is amazing.   Lately, I've been giving more thought to working full time freelance from home and I have some scary reservations.

The biggest one is lack of security.  My day job is very secure, pays me well (over 60k/year), gives me fully paid for benefits, and stock options.  I know it's going to be there every day.  I'm afraid about moving to fulltime freelancing without any guaranteed pay.  At any time, sites could close, I could be out of work.  I can't handle the thought of that!

Also, taxes scare me.  Right now, I'm leaving out 35% of my freelance income in a savings account. But I need to start paying quarterly taxes and I'm not sure how to do that.  I also don't know if 35% is enough.  I need to talk to a financial advisor and start learning how to pay quarterly taxes and how to save things for tax deductions.

And finally, health insurance.  I could get insurance with my partner, but that sounds like a hassle to get started and I have a great fully paid for healthcare plan through my employer.

Alas, finally freelance isn't ENTIRELY a reality.  I could theoretically do it with the pay from my new side gig, but what happens if it goes under?  I'm scared.

Tips?