Leni - you've been a mum in a million to your ds, you really have, and he's had 6 weeks of your bm which will have already provided him with all your antibodies and other good things. You really do need to start thinking of yourself too now, as he needs you rested and full of energy more than he needs to be ebf.
Bf-ing is one of those strange things that we all take to our hearts, and feel terrible about if we stop. I started out completely ambivalent either way, then settled into bf-ing, and was hit by a huge wave of guilt after dd had her first bottle. The thing is I can see that dd sleeps better after a formula meal, and has really bonded with DH since he started feeding her, so now I'm completely ok with it.
Think about yourself sometimes as well as everyone else. Being a mum is hard, very hard. We all start out battered, bruised and exhausted, graduate onto sleep deprived, chewed and spewed upon, and somewhere along the line succumb to endless levels of guilt. The thing we should remember is we are all stars, all doing an amazing job, and all need to make choices to make sure we do what is best for not just everyone else, but ourselves included.
Give yourself a break, a pat on the back, a big hug, and decide what works.
Vj - I've moved to mixed, and have done so gradually by cutting out first one feed, then two (but never two back to back). It does get a bit sore sometimes, in which case I express off just enough to be comfortable, but not enough for the milk bar to go into production over drive.
The thing to watch is the balance of fore and hind milk, as once you are on mixed, your boobs will not be fully drained down. DD had a few green nappies due to excess fore milk, so I've started expressing off the really watery stuff first thing and binning it. It makes me think I should fully transition asap as the one thing I don't want to do is mess up dd's nutrition.