28 is young. I'm 25 and married but yes I think it's still young. I feel too young to be married, truth be told, but I already have a child from a previous relationship and I knew a good thing when I saw it
If DH and I didn't have my DS then I don't think we'd be married yet. Although - on your last point, no, I absolutely would rather have not had my child than be stuck with the wrong person (thankfully, it's not a choice you have to make) but I think on balance I'd rather not have had DS at all than had him with the wrong person. I would rather be with the right person and not have DC (even though I think I'd always have wanted them).
But, I really don't think you're at this point? I suppose it depends how important sex is to you but to me, I think it just does get a bit mundane and tired in a long term relationship unless you're actually putting something into it (which I am quite conscious of doing).
I think Kougin is really on to something there. Missing someone makes them incredibly more sexy when you come back to them. My ex was really controlling and I never went out without him (not suggesting your BF is contrilling, BTW!) and the one time I did because my work night out didn't invite partners, I was really happy to see him when I got back if you see what I mean
and I'd been struggling to have any enthusiasm for sex at all.
Extreme example, of course - but in a normal relationship perhaps a little longer away by yourself would help?
Also a change of scene in general, ie going away together. That can be sexy especially when you're not thinking "Yeah this is nice and all but I came to bed later than I wanted to as it is and I have to be up early in the morning" - having the freedom to spend all day in bed is pretty nice 
And possibly just starting again? Play it like a sexy game, ban everything except kissing and holding hands, then slowly allow different things like nipple touching, etc whatever floats your boat. Or make it a secret where each boundary is so that you have to follow the cues of each other etc which can bring back the "is he, isn't he?" excitement. Or do it over the course of one afternoon or something where you have nowhere to be and nothing to do, have a competition to see if you can turn the other person on enough that they break the boundary and you can add forfeits etc. It's interesting because it makes you discover things you like other than the obvious old favourites and breaks you out of your "sex routine" which can get pretty samey.
And the other old Cosmo favourites
like trying a new toy/position/activity/new undies/shaving if you don't normally etc (TBH, some of these probably sound dullsville) but sometimes talking about random fantasies or things you've wondered about can spark something. Or even reminiscing about something you did back when you were first dating which was really sexy or exciting.