Hi,
This is hard to talk through in writing. Really, you need to hire a coach so that you can actually talk through your work with you. They'll help tease out where there is impact that you don't realize.
However, here are some tips:
1) Impact does not have to be purely quantitative - you can say qualitative impacts (like, resulted in successful launch, or accepted by CEO and implemented, etc. etc.)
2) You can provide estimate/ballpark numbers - if you don't have exact numbers, that's ok! I did some process improvements for a company. I know that their existing process took a few hours. I now that my new process probably would take a few minutes if done right. So, I put that!
3) You can provide " tangental" impact numbers - for example, I designed and built an onboarding process + application for our company. I have no idea if it was good or not! But I do know that we increased our staff by 50% that year (this was planned)...so I put that number! Would you feel comfortable estimating the size of the market for your go-to-market strategy? Put that!