是in my mind 还是on my mind?

是前者啦，习惯用法
be in one's right mind => right1. be in two `minds about sth/doing sth feel doubtful about or hesitate over sth 对某事物三心二意或犹豫不决: I was in two minds about leaving London: my friends were there, but the job abroad was a good one. 我对是否离开伦敦一事拿不定主意; 伦敦有我的朋友, 但是国外那份工作非常好. be/take a load/weight off sb's mind cause one/sb great relief 使自己[某人]如释重负: Paying my mortgage was an enormous weight off my mind! 我还了抵押借款, 如释重负! be of one `mind (about sb/sth) agree or have the same opinion (about sb/sth) （对某人[某事物]）意见一致, 有同样看法. be on one's `mind; have sth on one's `mind (cause sb to) worry about sth （使某人）为某事物担忧: My deputy has resigned, so I've got a lot on my mind just now. 我的副手辞职了, 所以现在有许多事要我操心. be ,out of one's `mind (infml 口) be crazy or mad 发狂; 发疯: You must be out of your mind if you think I'm going to lend you 50! 你以为我会借给你50英镑, 你准是疯了! bear in mind that... => bear2. bear/keep sb/sth in `mind

几个例子，参考下

on one's mind表示现在正在想的，比如，There's something on my mind.即，我现在心里有点事。

in one's mind有两种意思
一，“在我看来”：In my mind he is only a clown. 在我看来他不过是个小丑。
二，“在我心里”：You will always be in my mind. 你永远在我心里。

in my mind
表示我认为。
on my mind
表示使我担心

in my mind

可以选择jxia的说法。