Two possible lines of legal action should be considered to tackle the child maintenance set, apparently according to an agreement authorised in court. One option relates to cancelling the agreement, wholly or in part, according to the restricted grounds under contract law, to be examined according to the particular circumstances. In general it is very difficult to cancel court-authorised agreements. However, if you were not represented by an lawyer, issues of possible mistake or deceit, which are grounds for cancellation, should be checked out, as you do not speak Hebrew, before this option is ruled out.
The second possible option is to file for a reduction in child maintenance - based on a change in circumstances. On the face of it, from what you say, you could well have a basis for this - and you would be advised to get individual legal counselling, so that the agreement and ruling can be examined, in relation to the apparent changed circumstances of both yourself and your ex-wife. You will then be advised as to whether you have a good chance of succeeding in getting the maintenance reduced.