should be a doddle to get her over. At the time I brought my then GF over I'd only known her for 7 months. She was 19 I was 24. She didn't work and lived at home. I sorted her visit visa myself and it was accepted first time.
You need to find a reason that she needs to go back to Thailand and show proof of it if possible.
Self employed shouldn't be an issue. Just show proof of your income savings and your registered Ltd company I guess.
I put photos of my home in the application. I wrote a supporting letter detailing all the plans we had for her visit to see some different tourist sites in the UK and the wish to visit my family. I put letters in from my parents including contact details and they said that they were hoping to meet her. Include evidence of your 7 year relationship from the start. To show your relationship is genuine.
It's easy to do.
Google Thailand UK Forum it's a great site with loads of information
I brought my GF (at that time) to the UK for a holiday back in 2012. She had no job, I didn't put any money in her bank and was living at home. It is the responsibility of the sponsor to prove the ability to they can support them during the visit and provide adequate accomodation. I showed my earnings, my bank account and details of my home. The Thai citizen must show adequate ties to Thailand to show they will not overstay in the UK and in fact return to Thailand as intended.