By spillover, you mean your arms are brushing the girls, so they are a bit too low and wide? That means you need a smaller band size, and a larger cup size. The 'girls' should be brought within width of the torso, and from the side should appear to be halfway between the your bent elbow and shoulder. (So high and tight rather than low and wide.

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Having a smaller band size means the band supports you rather than the cups. The cups should just be for coverage, and the band should be below the girls, not touching them. Also, the band provides something like 80% of the support if worn correctly. (Cannot remember the exact percentage). The way you've described long straps and spillover, it sounds like the straps and cups are bearing the support. Not as comfortable, and not good for your back.
If you can go to a brick and mortor store that provides decent bra fittings, you can have someone help you find a regular bra that fits perfectly, then it will be easier to judge on you own when you order online.
Maybe this video will help? I could watch the entire thing from this computer, but I'll check back later.