If you're sending V5 problems and only 5.9's (or struggling up 5.10's) then clearly the issue is technique, NOT fitness.

I can't even boulder that high yet I can comfortably onsight most 5.10's even with a really heavy rack.

If you're bouldering at the V4/V5 level then finger strength is not a problem since you're dealing with really crimpy stuff at that point. What you don't get bouldering (or even in most gym routes) is delicate balance work.

Do this:
At the gym:
Practice going through a vertical or less than vertical 5.9 with minimal hands. You should be using just the friction of your fingers when possible. Practice flagging and high-stepping really close to the wall. DO NOT use your arms to pull.

Hey. I spend my entire summer in Florida and I really did not get to climb for the past two and a half months. I am on a team but it is only during the school year and I am four years away from being able to drive myself to the gym, so I really haven't been able to climb more than two or three times a week. During the year, I have been able to climb 5.10s pretty normally and I am projecting V5s for bouldering. Now I am slightly struggling with 5.9s and I am having problems with V4s. I am about 5'2 and 100 pounds. I am in pretty good shape and am not fat. I know I just have to climb and climb and work back to where I was before. For strength, I have climbed up and down 5.8s and 5.9s but have not overworked myself. I was wondering if anybody had any tips or ways to get back to where I was before? Thanks a lot.

I'm going to have to disagree with you here...

From OP's post (4 yrs away from driving) it sounds like (s)he is 12-13 and 100lbs. At this age and being in good shape getting pumped should not be an issue. Especially considering at the V4/V5 grade the holds will be WAY smaller than almost anything on a 5.9.

Moroever, climbing 2-3 a week over the course of a year is more than enough to build sufficient endurance to climb at least 5.10b or so. Given that gym routes are heavily overrated this leaves me with no doubt that endurance is not an issue here. Unless, of course, OP's gym has multipitch 5.9's

Notice I suggested practicing on a VERTICAL *OR* SLAB route. Flagging on a slab is silly. However, learning when to flag on vertical routes significantly improves movement efficiency. No-hands climbing on slabs is not route-specific training (given OP's level) either. The delicate balance-work learned here will serve OP on any vertical route.

Focusing on what kind of 5.9's OP is climbing isn't really helpful either. At that level (especially gym 5.9's which are more like a 5.7) overhanging routes will be juggy and vertical routes will have plenty of small but manageable holds. My guess (and I'm almost certain this is the case) is that OP is treating each route like a boulder problem and muscling through it.

The question was, he used to boulder V5 and rope-climb 5.10, but now struggles with V4 and 5.9s. And both of those have more to do with the 2.5 months break than anything else.

The fact that he boulders harder than he is climbing on the rope could be the result of not spending enough time on routes. A young kid going to the gym, even 2-3times a week, would probably need an adult to belay. Even the most-lenient gyms that have gri-gri-s bolted to the floors, and allow young kids to belay, set the limit at 12 or 13yo. It is much easier to have team kids bouldering with one-two coaches, than to have them rope up with one-two coaches and get adequate time on the ropes, even when they do rope up.