I think you have a bit of a phobia about being penetrated, you should relax a little more mate, its just another part of your body.
When i first experimented i just used a finger with lube, i found it quite a turn on, but i didn't like anything bigger for many years, until a couple of years ago when i became interested in tgirls.
I thought if i ever meet one i better get ready for something bigger than finger.
So i bought a vibrator for my girlfriend and stated to use it on myself secretly.
At first i couldn't get more than 2" in and i didn't like it much, it was a bit pailful and strange, but i persevered now i can bury 6" and really enjoy it.
You got to use plenty of lub and take it slowly over the weeks, in the end you will probably love it, just relax about it mate.
Take it like a man
:D:D
Maby you need to focus on yourself a Little bit more as well when your with someone, most of the girls ive known get a big kick knowing thay are giving you pleasure as well as receiving it and it doesn't mean your being selfish as long as its not all you.