Hey Xestaro!
I began Muay Thai, recently, so not super expert still, however I'm tall as well (1.86) so I have quite the same challenges.
My jab however is longer than my cross, as I don't over-extend the cross when punching it (unless I see a clear opening).
First thing to work on is footwork I think. Make sure you are slid on your feet when punching. Think of your body as a tree, if you are not well deep on earth, you won't grow your punches. So work on your footwork, don't concentrate too much on power yet, but more on matching your feet with your punches.
Second thing, make sure your elbow don't leave the axis of your body on your jab and cross. It has to be a very straight line. It works very well for me for both Jab and Cross when sparring, as I can land them quicker and more accurately.
Third thing, rotate your hips slightly on your jab, and much more on your cross.
Do a lot of shadowboxing to get the basics, then go on the bag but don't go full power.
Hope it helps!