Right. heres what it could be...
Your partner: Sure, you love her, thats great. Good for you. Doesnt mean she is great in the sack... Or even if she is, maybe you just dont gel well in bed... either dump her or stck through it trying to spice things up with kinkyness..
You: Your cumming too soon/not too good in bed/etc. If this is the problem, chances are you deny it to yourself or plain not see it. Try desensitizing yourself (dont ask me how, google exists for a reason), or just practice to make perfect. Hopefully its not you though :P
Its just the sex: Sex has become boring or routine. Try some fetishes. More foreplay/less foreplay, Anal, spanking, biting, femdom, just find something that gets you both cumming buckets...
Hope I was a little bit helpful. Not claiming to be a sex god, or even get a ton, but im just making obvious suggestions.
Go forth, fuck, and be jolly ; )