Replace the tire definitely. The rear wheel is much more of a how you feel on it kind of thing. Structurally it will be fine, however the biggest concern it that the gouges have made the top of the rim rough and liable to cause damage to a new tyre you put on.
I'd take a fine gauge file to make sure that the outer round edge of the rim is smooth, then as
@I like Skol said, get some emery paper (not sandpaper as it's too rough and not intended for metal) and just finally smooth the rim out. Do the same (emery paper not file) on the braking surface until it is smoothed out. You don't need to get it perfectly smooth, just no longer sharp in any place, if you can run your finger along it and feel nothing grabbing it, that's fine.
I wouldn't worry overly much about your brake pads - they last long enough and are cheap enough to replace - but they will improve your braking no end, I found Tektro pads quite hard and lacking power in the wet, which won't have helped your confidence any.
As
@I like Skol also said, if you are locking up your rear wheel, your braking technique needs work. A lot of the time people tend to over-use their back brake because they are afraid of going over the handlebars like we often did as kids, but that's the wrong way to use your brakes. As with motorbikes, about 80% of braking power comes from the front brake, there are lots of good articles explaining why this is the case, so if you're interested I can post one or two. But nevertheless use your front brake most of the time.
When braking for a corner, you should aim to do all your braking
before you start turning, braking in a corner is a recipe for the rear wheel in particular to lose traction and slip. Also watch out for patches of oil or diesel on the road as these are murderous on a corner or roundabout.
Lastly, you don't say what type of tire you are running, nor what size, and how heavy you are. It's unlikely that having the tire pumped up to 100psi (I run that sort of pressure all the time) will have caused your wheel to slip, but running a lower pressure may well make the ride more comfortable nonetheless.
If you let us know roughly what you weigh, and how wide your tires are (you can tell by the markings on the tire itself, you will see something like 700x25c or the like, the 25 is the width (25mm in this case)), we can advise on a good pressure to run. It is also possible that your tires are not particularly great in the wet - different tyres have different rubber compounds which behave differently, a stickier compound is better in wet weather as it grips the road better - but it does wear faster. Different tread patterns make no difference to how a road bike tyre performs in the wet, so slick tyres are just fine.