In addition, infinitives and participles differ in the way in which they express the reality of an event. For example, "to. . ." tells us the event hasn't yet happened; it's unrealized, whereas ". . .-ing" tells us the event has happened or has been experienced before; it's realized. So, "Making mistakes is easy" means the speaker has made mistakes before, or knows about making mistakes, so the event has happened, has been experienced. In contrast, "To make mistakes is easy" sounds odd. It expresses that the speaker has not experienced the event before (How does s/he know it's easy? ) That's the awkward bit.
1. "Selling insurance is a pretty boring job."
=> The speaker has experience selling insurance or hasn't but has knowledge of it (i.e., someone else does it and told the speaker about it).
2. "My ambition was to retire at thirty."
=> The speaker intended to retire at 30, but s/he never did. The event is unrealized. It didn't happened when s/he was 30.
*My ambition was retiring at thirty? (ungrammatical)
=> It means, my ambition was in the act of retiring when you were 30.