A few thoughts:
1) start by rubbing your precum on your lips and licking them clean
2) when you get bigger amounts of precum place it directly on your tongue to taste it, see if you like it
3) edge to the point of cumming, back off and stop yourself, repeat until a small bit of semen escapes and play with it (smell it, rub on your lips, lick it off, place it on your tongue to see how that feels, look at it in your mouth and on your lips, etc.). Get use to having semen in your mouth.
4) when you are use to the smell, and feel of cum in your mouth (and on your lips, face) take the next step and swallow these small starter amounts.
if you still can't make yourself taste and swallow your cum and are really sure you want to try it:
5) buy an "O" ring mouth gag with a small lock
6) when you are really horny and want to try legs over head again place the "O" ring in your mouth and secure the gag with the small lock (be sure to practice being able to unlock it behind your head with your key a few times first)
7) with your mouth locked open assume the position and shoot your cum in your open mouth
8) if you can't shoot in your mouth yet, try aiming at your face and see if you enjoy having a self facial (make sure to look at yourself in a mirror afterwards so you can enjoy the visual)
Bottom line: you need to get use to playing with your semen on your face and in your mouth and curiosity, desire and or lust will have you finally able to taste and swallow your first load. If not, you'll have an accidental orgasm along the way while playing and inadvertently get cum in your mouth at some point, ready or not!