Waswas must be addressed by a believer. The way to address it, as you have stated, is to ignore the doubt. When you get such thoughts, just say to yourself: no this is Shaytan making me doubt my intention. I am praying sincerely for Allah and I am not showing off. Then ignore the doubt and continue your prayer. If you do this several times you should overcome the Waswas Insha’Allah.
Also, when you start prayer, with your right index finer poke your left thigh and say:
بِسْمِ اللَّهِ وَ بِاللَّهِ، تَوَكَّلْتُ عَلَى اللَّهِ، أَعُوذُ بِاللَّهِ السَّمِيعِ الْعَلِيمِ مِنَ الشَّيْطَانِ الرَّجِيمِ
This is effective in removing the Waswas Insha’Allah.