You say "I find it harder to have sex because of this", but that does not make sense! Harder than what?
Since you have never been any other way, you cannot
find it "more" difficult without anything to make the comparison. What you are saying, is that you have some small concern about having intercourse, and you imagine
that something you identify - not being able to fully retract your foreskin - is responsible for your perceived difficulty.
But in fact, whatever it is that you find difficult - and it would be interesting if you could explain just what that is
- may not have anything at all to do with inability to retract. Particularly
so if you have no difficulty masturbating - and I am sure you do not - a foreskin that will not pull back is generally an advantage
in masturbation as the pressure - with a reasonable degree of retraction - is pleasurable in itself.
Now in fact I am not at all saying you should not
proceed to undertake the stretching "exercises" and achieve a smoothly retractile foreskin, and it probably will
be somewhat more comfortable and pleasurable for intercourse, but simply that you should consider other
reasons for actual difficulties.