Rate rise negotiation - need hand holding!

[MrPickles73]

I work free Lance and have had the same client full time for nearly 2 years. When I started it was temporary and my rate was low as the market was poor in my industry.
In 2 years they have refused to increase my rate. I told them at Xmas I would leave at end of march if I could not agree a satisfactory rate rise. (I was tired and cheese off at the time. The market is still not great but better than 2 years ago).
2 weeks ago an old client approached me and said they has some work for me ASAP but could not guarantee a period longer than 3 months. They have offered 10% higher day rate than my current client.
I have told my current client that I will need to give them 1 months notice from tomorrow if they cannot match the other company's offer.
I feel I am going out on a limb here on principle but at the same time I feel strongly that I am being underpaid. Am I crazy to throw away my existing more reliable but lower paid contract for slightly higher pay but less certainty?
I feel I have reached the point of no return!

[MrPickles73]

.. just in case anyone is in a similar boat.. current contract offered me and slightly different deal re expenses but only 5% increase so I have said no, put in notice to terminate contract etc. It seems existing job is stunned as I am at a crucial time in.the project...

[Hoppinggreen]

It’s about knowing your worth
If they are underpaying you they are undervaluing you
I never negotiate, I State my rate and it’s a yes or no. Usually a yes

[heidivodca]

Stick to your guns and go for the bigger increase - good luck in your new role. As a freelancer always go for more money!

[heidivodca]

I bet the new job lasts more than 3 months (my last 3 month rolling contract would have gone on for at least 18 months had I not left after a year for a better contract elsewhere (again on more money! Hoping this lasts another year or two - seems likely atm!)

[Hoppinggreen]

My last 3month contract lasted 9 and my 4 day “one off” project is into its 3rd month!
If you are good they usually find the budget