I suspect that the fear of downhill is mostly just lack of experienc because you've been avoiding uphill, so you'd probably acclimate yourself to the higher speed if you put in the time climbing. I used to ride in San Franciscco a couple decades ago, and was amazed at how fast I got used to riding down steep hills in traffic.
I love hills, except when it's hot and humid, at which time I will tend to avoid them.
This might not be popular advice, but it works for me--do everything at the highest gear you can tolerate. It may not be the fastest way to go (at least at first), but it builds leg strength like crazy. Running at lower gears may be better for cardio, but you don't build as much muscle. Since muscle to weight is the key ratio in determining your speed going uphill, it makes sense to build more muscle as well as lose weight to work on both sides of that ratio. But please, if this makes you miserable, don't do it, my regimen is not for everyone, and you don't want to ruin cycling for yourself by doing something unsuited to your needs.