I think if you want a little more force behind the ejaculation, the best way to get there would be to work your PC muscles a little each day-- Be careful though, it's like working out any other muscle and if you get too gung ho with it one day you'll know about it the next!

As for size, I think too many guys are obsessed with it. This is one case where it seems like you are fine with yourself, but your girlfriend might not be ok with you. Either way, it is your cock and her issues- the only ways to change that are surgical, unpleasant, not very efficient and downright dangerous. I think that part of your situation may have to end with a change in mindset (hers) rather than body (yours).

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I am certain of nothing but the holiness of the heart's affections and the truth of imagination. What the imagination seizes as beauty must be truth - whether it existed before or not.
-John Keats