How can I stay motivated to lose a lot of weight?

I’m a 42-year-old woman, and I now weigh an embarrassing 340 lbs. I'm determined to lose weight without surgery, so I’ve been on a diet and exercise regime for the past 6 months - but I’ve lost less than 20 lbs. The fact that I’m losing weight so slowly, and that my regime is so difficult to follow, has really deflated my spirits. I’m now at a point where I’m ready to give up and stay fat forever. Please can you share some weight loss tips and ways that I can stay motivated to lose all this excess weight? Thanks in advance for the suggestions.