I really wish my wife hadnt come on here...!!!!!!
Serious note, this is a genuine problem for some couples and ofcourse you will get a proportion of sniggers and innuendo on this site. Some of the advice thus far is good.
Not to blow ones trumpet but I have had this problem myself with a couple of partners. I think it depends on the individuals concerned. I have found patience, honesty, variety and lubrication is the key. If you love each other you will overcome this. Sex is an important aspect of any relationship and you say yourself it is super enjoyable most of the time. Trial and error and communication, you will be fine. I think as males we can get pre-occupied with "size" but it can be a hindrance.