The best angels and seed investors can often serve the role of a mentor quite well, especially if they were CEOs or had similar operational experience.
Where it breaks down is using a Series A-B-C investor as a mentor. While they will be highly engaged in your business, it’s also tough to truly share all your struggles and challenges with someone that has invested many millions in your company. They are simply going to worry too much about the investment itself.
If you can, though, turn one of your angels or one of your first, earliest seed investors into a mentor. Someone you can talk to at least 2x a month about everything on your mind. See if that works. See if the feedback is helpful and defining. Finding a great mentor is tough. If there’s already one on your cap table, that can be magical.