Pretty good advice from Bryan. I'd take a couple of exceptions to what he said, but pretty minor at that.
Like he said, the E. Butt of MCR is really pretty straightforward: you've got maybe half a pitch of 5.9, a short bolt ladder you can aid if need be, and the rest is 5.7 and 5.8. It's is far easier than the Serenity/Sons link up. It would probably be a good test to see how well you move on a longer climb before dealing with a bunch of consecutive 5.9/10a pitches on S/S. I did the E. Butt during my first week of climbing in the Valley. The climbs I did beforehand were Royal Arches, Harry Daley route and the Jam Crack. Though we aided the bolt ladder, nothing on it felt really hard. The 5.8 pitch high up is pretty solid for the grade, and there's a 5.7 pitch without much pro (but pretty secure feeling), but it's all there. One caveat though, I had done alot of trad at Josh and Idyllwild and so felt pretty comfortable leading 10a and bs.
I'd check out the following routes to progress through the grades. Keep in mind that the Valley can throw all kinds of interesting stuff at you, even if it's "only 5.8." Since I'm at the office and away from my guide, this is just off the top of my head:
Positively 4th St.
Reed's Direct (1st two pitches)
Central Pillar of Frenzy
Maxine's Wall (1st pitch)
first two pitches of Serenity
Outer Limits (1st pitch)
Gripper (I know it's supposed to be the standard for .10b, but the short crux feels hard)
Stone Groove (haven't done it, but it looks good)
Chingando (most people TR after doing the Iota (5.4))
1st pitch of Salathe
Five and Dime
Never got to the base of Sentinel but these are supposed to be awesome:
Contrary to what's been said, while I don't advocate it, you can take falls on your typical Valley crack. It should be vert or pretty close to it, and you can stitch it up with pro at your waist if you choose. You never know what you're capable of until you push yourself, so pro it good and push yourself.
Having said that, you're biggest problem appears to be your head. If you've TR'ed hard .10s (and climb .11 indoors) and you've only managed to do a couple of 5.9s, there's a big disconnect between what you can do and what you're willing to do. One mental trick I've used (though only for face climbing) is to TR something hard and then hop on your intended lead. If you've just TR'ed a .10d, I guarantee that .10a/b is going to feel pretty mellow in comparison. Once you've convinced yourself you can climb that hard, you won't backslide. To use myself as a comparison, when I was 15, my hardest lead was a 5.8. I knew I could climb harder, so I jumped on a 5.9 and after doing that tried the .10b next door. I never slid back (though there's always that weird 5.9/10a that will mess with you). From what you've written, I KNOW you're capable of making a similar jump.
Other things to try are bouldering, working on harder stuff but also working on climbing with control, possibly even statically, just as you might on a lead. If you can boulder, say, V2 or V3 then, again, 5.10a is going to feel light. I see lots of people coming out of the gym climb as if they're on toprope or only two feet out from a bolt--sloppy, slapping for everything, etc. You just can't do that on a trad lead. Climbing in control is a learned skill, but lots of people don't want to bother with it because they seem more interested in pursuing big numbers, not style. You can also find some boulder cracks to TR. The guide has a few: Dynamite Crack (10d thin), Monster Crack (.11a hand/fist). There's stuff at Swan Slab. Get the technique down, learn to work stances, shake out, milk a good jam for a rest, etc.
Personally, I don't think the first pitch of Serenity is anywhere near as tricky as the crux, which is why it used to be rated 5.9. Even now it's still considered 5.10a, but barely. The second pitch feels much more true to .10a. The first pitch pros well too, contrary to what some claim. I think the .10d IS tricky, in part because the crack fizzles out near the top, so you can't necessarily beef your way thru it.
My plan would be to do a fair number of 5.9s and 10as. You can probably do either Central Pillar or E. Butt, depending on which you feel like or can get on first. Do a few more harder climbs. If you can lead Crammin and set up a TR on it, you'll be plenty strong to do Serenity. Personally, I thought the crux of New Dimensions (.11a) felt easy after doing Crammin. It's great to set goals, but be open to the enjoyment of all the routes you'll climb en route to your projects. Years later, they're as vivid in my memory as the bigger things I climbed afterward.