Looking at it backwards...
こうしたらいい
If I do this, good outcome.
こうしたらよくない
If I do this, bad outcome.
And...
こうして(は)いい
I can do this (because it achieves a good outcome).
こうしてはだめ
I can't/shouldn't do this (because it achieves a bad outcome).
So it kind of sounds like...
どうしたらいいか分かりません
I don't know what will achieve a good outcome.
どうしていいか分かりません
I don't know what I can possibly do. (OR...)
I can't figure out what I can/should do.
That's my best attempt at clearly capturing the difference between the two, but if you're still unsure, I do think that considering the feel of こうしたら? versus こうしては? can illuminate the difference somewhat.
I also feel like どうしたらいい is a little less immediate/urgent than どうしていい, but I'm not sure how to explain that difference well.
The difference is difficult to pinpoint, really...but I imagine it won't make a great difference most of the time, either.