So a person goes through waswaas quite a lot. Alhamdulilah, they felt like it is reducing now. But still thoughts about Allah (SWT) comes. The person is not sure now, if the thought is from themselves or shaytaan. So how do you DIFFERENTIATE thoughts?
Not sure what I write will be fully on-topic but just wanted to help.
There's no way to differentiate these unless one displays the severe symptoms associated with an all-out demonic possession which is obviously not your case.
But to combat these thoughts, you don't have to try chasing them out of your mind or feeling guilt or getting overly stressed. These reactions are completely counterproductive. Instead you distract yourself by engaging in productive work, socializing, physical exercise, praying in congregation, doing the housework, taking a bath, etc. Choose those hobbies in particular that are less stressful and cause less mental burden. You want to do them with convenience, enjoy and relax.
If these don't work, you will then certainly need to consult a physician. There could be problems with your diet, life habits, or your overall psychological health. Only a specialist can tell.
wa huwa yashfin...