I posted elsewhere a while back but this may help with your journey once you get launched:
Supercruise
The solution to your experience is to not think of your journey in straight lines.
When in Supercruise a straight line, or the shortest route, to your destination is not always the quickest!!!
As others have said being close to planets etc. slows you down. The gravity of these bodies prevents your drive from reaching higher speeds. The higher the gravity the stronger this effect is. Being close to such a body is called being in its "gravity well".
The quickest way to get anywhere when you are close to such bodies is to get away from the gravity well first, even if this means heading away from your intended destination! When you are free from this gravity well you will see your ship rapidly increase in speed.
Once this happens steer towards your destination but take care to avoid the planet you have just escaped from.
Of course when you approach bases that are in close orbits to planets you will not be able to evade the gravity well entirely but by approaching from behind or from the side you can avoid/minimise this effect until the last moment.
The pictures show how a gravity well works. If you think of this "well" actually as the hole it is shown as then all you have to do is avoid "falling" too far into it to avoid being slowed down (having to climb out).
I'm hoping this terrible picture I made in my 3D software helps to explain this.
It shows a planet and a moon (dark spheres), around them is the gravity well (translucent blue sphere). Orbiting each body is an orbital station (grey box).
There are two routes. The red line is the shortest route in terms of distance and the green line is the longest.
However the red line cuts across two gravity wells and will take the longest amount of time to complete. The green line in this case is by far the quickest way to get from one orbital to the next.