I've been following the guide for the checklist mission of flying the DG to the ISS but I'm consistently getting into trouble at one particular point. I'm able to get up to the point where I've tuned NAV1 and NAV2 to the ISS but getting my relative velocity to zero is nearly impossible. I've been needing to either engage my main thrusters or use retro thrusters to get it close to zero but it certainly doesn't stay there. At best, I've been able to get myself within 10km of the ISS but ultimately I can't reach it and all the effort to get relative velocity to zero throws off my orbit and I end up falling back to earth. What can I do to remedy this?