How can a non-native English speaker get assistance in publishing a manuscript in English? I am not a native English speaker, and I am writing a paper in English (maths) with the aim of getting published in a peer reviewed journal. If I write the manuscript myself, the poor grammar would prevent it from being accepted. If I give the manuscript to a native-English speaking expert in my area of maths, they may steal my ideas. If I give the manuscript to an expert just in English, they  won't be able to fully help because they won't understand the ideas comprehensively.

Would someone please guide me how to solve this situation?
In mathematics, it is entirely reasonable and acceptable to begin by posting a draft on arXiv.  Once it's up online with your name and a date-stamp, you need not fear anybody stealing your ideas, and can work on improving the manuscript, both in content and grammar, with whatever combination of colleagues and collaborators you find best suited for the task.