Hate to see you give up on the sport. There could really be a lot of factors. How long have you been cycling? Conditions for the ride? i.e. Very hilly? Winds? Training preparation etc...
As far as gearing - a lot of that is feel, but if you use a cadence meter that will certainly help. Use it the same way as you would the RPM on a car and when you would shift. When your cadence begins to drop, due to wind, terain, whatever, drop to an easier gear. Just stick with it.

I'm still rubbish :)

Though not as rubbish as I was when I started. It definitely takes time to build cycling strength and endurance, seems especially for runners. If you stick with it, put in the miles, and try to do some focused workouts a couple times per week you'll eventually get the hang of it and improve.

My experience has been that there is a "beginner's" plateau, by which you suck for a year and then the miles catch up with you and you make some headway. I took up cycling when I was about your age and gave up for the same reasons. I took it up again about 5-6 years back and wish someone had told me that if I stuck with it back then I'd break through that plateau.

On the cadence and climbing, I that's my weakness as well. i'm fine on the flats and descents, but even the gradual climbs tend to hurt me compared to my peers. So, this year I'm trying to focus on doing gradual climbs at least twice per week. I try to keep my cadence up, but also drop it down (as virturace mentioned) to work on building strength.

So, bottom line is yes there are many like you out there and it just takes persistence and mileage to improve. Don't give up on it, you're young and cycling fitness is a continual building process year after year. You could be very strong in 4-5 years if you stick with it.

Just as an example: Both my brothers never rode bikes before taking up the sport, aside from some tooling around on a bmx. After about 5 years of persistent riding they've each built a reputation for being top cyclists in the sport.

I'll agree with tribro (at least in my experience) - I had that beginner's plateau. I started really getting into cycling about a year ago. I started riding with a group of guys at my work. In the beginning, they kept it a little easier for me and once I got the hang of riding with them, they started to push it more. Now, there's this hill we always climb that's about a mile long on our route. Always, always they would beat me up it and they would have to wait for me at the top. One day this winter (in the first month or so of my Half Ironman training), I broke through this plateau and beat them all up the hill. Something just clicked. I'm not sure what - but from that day on, I've continued to see improvements in my riding ability - climbing, speed, endurance, etc. And, my beating them up the hill that day made them reassess their riding and training and they have since become better climbers.

If you love to ride, don't give up. Who cares how slow you may be right now? It's all the more reason to keep it up and prove to yourself that you can get faster. Once you start seeing results, you'll be glad you didn't give up. Now, if you hate cycling, well.......

Have you been riding with other people? Riding with people better than you will help make you a better rider - you can see their technique both on flats and climbs and find out what cadence they're most efficient at and see if you can mimic it.

Good luck!

well i enjoy it, but the way i see it, if i focus on my running id be able to do some hot times, where as in triathlon ill always (well for my next few) be near the back which hurts for a competitive guy, on my first tri i came near the top run and swim but because of my poor bike leg i was near the back at the end, but ive taken heart from what tribro said, and hopefully in a few years i ll be a proper triathlete not just a runner with a bike :)

Great tips... I still feel way more awkward on the bike than either running or swimming. As far as cadence goes, I have a bike comp that I love (Forerunner 301, also for running, etc). What cadence meters do you guys recommend? I really only need something for cadence as the other one does everything else I could want.

That is also the allure and challenge of tri's.
We all have our weaker sport. For me it was both running and swimming. I'd pass everyone on the bike that left me behind on the swim only to have them start catching me on the run. This year my run has broken through a plateau and it is nice to see all the hard training paying off. Making good progress on the swim too.
Use your competitiveness to challenge yourself to stick with it and put in the effort needed. It is worth it.

DJTubs - you say that you're competetive, but it sounds like you are only so when you feel that you can win. Part of what keeps this sport from growing (which may be a good thing?) is people's fear of failure. True, it's always more fun when you win, but how much fun is it if you always know that you're going to win?

I started this whole tri nonsense in January. My first splash and dash, I came in 70th out of 100. Talk about bubble bursts! I was a top-notch wrestler in high school - not used to losing, certainly not used to losing to some of the people that I watched cross the finish line ahead of me. But it was so unbelievably motivating.

Last week, in the same race, I took 15 and two days later finished 17th overall in the Rio Salado Triathlon. I was 9th in the swim, 9th in the run and 27th on the bike - I suck at it too! My point is, that you can give it up and win some 5Ks - drink a cup of beer when you're done and call it a day, or you can fight like hell to find your cycling legs and use your other skills to keep you in the race until you do.

Oh, and by the way, you still have 12 YEARS before you start to hit your peak as an endurance athlete! I started when I was 33 - I would give anything to have started when I was 18. You have no idea how luck you are.

Cycling is built on a different chassis, even tho' you're using the same engine as in running and swimming. At younger ages, muscle fiber density is such that dramatic improvements can be made in short order. A few weeks of increasing cycling mileage, will develop the muscle group that powers the pedals, and your lower legs will look very defined as well as increased in mass. Go for it.