R’ Yisroel Salanter zt”l explains that there are two main opinions regarding Bitachon:
The Chovos Halevavos maintains that you must do hishtadlus along with Bitachon Not that Hakodosh Baruch Hu can’t do miracles, rather we just don’t rely on miracles.
However, the Ramban is of the opinion that Bitachon is to rely solely on Hashem without any additional effort.
Even according to the Chovos Halevavos, it is still forbidden to do too much hishtadlus, you may only do a small amount so that whatever Hashem sends you looks like it is “natural” and not miraculous.
But it would be a complete lack of faith in Hashem to believe that some people are just “lucky” or that the only way to succeed is to work as much as you are able.
The Alter adds, that it seems that the way of the Chovos Halevavos is for ordinary people but those who want to have a special connection with Hashem choose the way of the Ramban. The Chovos Halevavos also mentions that there are individuals who work very hard on their middos and are supported without any effort.
[It is possible that even like the Chovos Halevavos, there is no specific requirement to have bitachon with hishtadlus, rather there is a chiyuv to have bitachon, and for those who are not holding by not doing any hishtadlus they must still have bitachon with hishtadlus until they are able to have without.]