Today I really struggled not to snap at her
What's the worst that can happen, if you snap at her?
If she’s in a good mood our day is fine but I’m constantly on top toe, like I was in my marriage.
She probably senses some of this. Your hesitance in calling her out, small body language cues that denote a person who has been bullied previously. For some people, that;s like blood to a shark.
So you need to stop hoping she'll be in a good mood, & start putting her back in her place. Which is a professional colleague who needs to start acting like one.
This will help - give yourself a present & order a copy - www.amazon.co.uk/Woman-Your-Own-Right-Assertiveness/dp/0704334208
You need a couple of go-to phrases for when she next Smart-Alecs or finds fault.
"Monica - I've been giving you the benefit of the doubt as you lack experience - but your attitude hasn't improved so I'm going to have to ask you to observe basic professional courtesies in the workplace. I need you to stop doing XYZ & stop saying things like ABC. I hope I don't have to raise this with you again. Now - are those invoices ready, or do you need to blah blah subject change"
"Monica, it's rude to speak to colleagues in that manner. You need to stop doing it before it gets you in bother." blah blah subject change
"Monica, if you keep giving out with that attitude, people are going to resent you & start complaining to seniors. I'll let it go this time, but don't speak to me - or any colleagues or clients - like that again, ok?" blah blah subject change
The subject change is important because it draws a firm line under the conversation & indicates that you are not interested in debate - just in getting heard. So if she pushes back - "Your attitude is increasingly unprofessional. I am doing you the favour of allowing you to improve it, instead of raising a complaint. You might prefer to take that favour than to keep back-chatting."
Remember - you have 30 years on this young woman. Your experience counts, your right to professional courtesy counts, & your maturity can act FOR you when you cultivate an attitude of - effectively - (never say this bit out loud!) "you silly little bitch, you are shooting yourself in the foot here & have picked the wrong woman to bully."
Also - of you do need to speak to her along the lines of the above suggestions - tell your boss you have done so. An immediate email would be your best bet, to cover your arse (& ensure you get in first).
Keep it as simple as possible, but something like -
"Hi Boss
No action required but want to give you a quick heads up.
I've taken the opportunity to correct Monica about her attitude today. This has been a small but ongoing issue for a while, which I put down to relative inexperience, hoping she would settle in & pick up on our usual culture.
However, her rude remarks have ramped up to the point where I'm concerned that she might have the same attitude toward others, including clients & suppliers, so I have asked her to stop making them, & advised her that she needs to address everyone with professional courtesy.
Am hoping that will give us the required result & you won't need to engage. Happy to give you a quick verbal overview when convenient so you are in the loop - but with luck we will see improvements & you need not get involved.
Kind regards
Spice."
Note the pronouns - where I use "WE".
This sets you apart as the co-senior (rank not important - experience & age is) who is working WITH her boss to prevent problems.
Rather than the complaining old harridan this young woman will paint you as if you let her get in to your boss first ... 
It's a lot to think about if you are a naturally diffident person. But your only other choice is to tolerate it, & that will grind your self-esteem so low that she'll end up running rings round you. So take your time - maybe read the Dickenson book, & have some role-play practice with a trusty friend before tackling this rude young woman?