I have tendonitis in my back, so it can get very sore after riding.
Can anyone help me to adjust my bike so it can help to relieve some of the pain? I have thought about getting a totally new bike, but I'm not sure I want to spend the money. Right now what hurts is bending over to reach my handlebars.
Yes, professional help can do wonders for you. It sounds like you need to raise the bars up and maybe bring them back towards you by putting on a high rise stem. Different frames will have different geometry of the head tube angle, top tube length, seat tube angle, all these things will have an effect on the feel and your position on the bike. You may also need to alter the way you ride. But again, professional help may be required for that.
Your problem sounds like mine except it was my neck that killed me unless I rode sitting up nearly perpendicular to the ground. To achieve that on a road or mountain bike would have required an extremely long stem extension or else not having proper leg extension. I changed to a comfort bike with high-rise bars and now ride in comfort, though wind and hills are more of a problem. At my age the comfort bike is safer and more practical.
What part of your back hurts? And what kind of bike are you riding (hybrid, commuter, road, mtb, etc...)?
I have a real bad lower back and shoulder so fit is everything for me. I still experience some pain after long rides, but I don't think that will ever change. My bike fitting certainly reduced the discomfort though.
I agree with everyone else, getting a professional fit could mean everything in the world.
The prescription for tendonitis is usually stretching. Are you getting it treated? I think too many people throw out the "get a professional fitting" advice way to quickly. If it's that complicated to get the bike to fit right, there's something wrong. But you should at least consult a physical therapist to find out what you can do to alleviate the problem. That in itself should point you to what you may want to do with your bike.
I won't say the "R" word because tendonitis is a temporary thing and buying a new bike is more long-term.