Claire, your little rose bush is beautiful. It needs to be pruned and flowers need to be removed. What have you added as fertilizer?
First, the basic thing to understand about flowers is that they are temporary, meant to attract pollinators. Once the flower has been pollinated that flower starts dying. It has done its job.
Reproductive growth takes lots of energy, carbohydrates made by the chlorophyll in the leaves and some stems. Fertilizer is not a 'boost' Fertilizer is not food. Fertilizer added in just the correct amount (balanced plain old NPK) not too little and certainly not too much is critical for all plants according to their own specific needs.
This rose is showing some weird symptoms; top of the plant looks healthyish and the bottom looks anemic or has been blocked from sunlight. Need to know what you've added.
Get used to deadheading all flowers. The basic principle is flowering takes lots of energy and if you aren't in the seed making business cutting off those flowers as soon as you can stomach cutting off flowers, is the best for your plant. If this were an annual, an annual having but one goal in this life- to set seed-once seed is allowed to set that plant actually starts dying.
Your rose is a perennial but the basics still apply. A plant that has set seed now has to use more energy abscising the seed, useless leaves and stems. So if you cut them off sooner again, you are saving energy expenditure for your plant making it healthier, more robust and able to fight off disease and insect problems.
Black spot is a great example of a plant taking care of a disease of fungus. The plant sequesters, quarantines the fungus by cutting off the area infected by the fungus. Looking at it under a microscope is amazing. Surgically precise separation between the healthy leaf and the infected leaf inside the circle or boundary. Cut off from water and chemistry (versus 'nutrients') the infected part of the leaf drops out of the leaf, causing a 'hole' in the leaf.
Black spot is common and can be found on the healthiest roses and many other plants. A healthy plant is able to take care of itself. Adding fertilizer, not compost, is as important as light and water and drainage. Compost is for soil tilth, not fertilizer or soil replacement. "Less is best More is death and None is dumb"...ugh, my little ditty about adding balanced fertilizer, it might help to remember!
Use alcohol to clean your 'by pass pruners' (don't use anvil type pruners), even good kitchen shears are fine, but clean with alcohol first. Snip the lesser branch from its host branch or the main stem/trunk as close as you are able. Try not to leave big stumps. Remove branches that are angled into the side of your home. Remove branches that are so buried under other leaves that their leaves are unable to receive any light for photosynthesis thus allowing other leaves to do a better job for the entire plant.
Start with one or two branches and take pictures to send to us. Remove the weight of the 'trusses' of flowers...ahhhhh...from a branch and watch the branch bounce upwards again.
Flowers die. The first sign is fading, petal fall, blackening of the center and the making of seed is underway. Major energy expenditure. Cutting off flowers sends all that energy of making seed back into your plant to make more flowers, more lush leaves.
Has this plant been moved recently? Or was something moved away from this plant?
This rose must be a climber or a spray mini rose. I think a support might help it an awful lot. The more air you can keep going over the leaves of your plant the less you need to worry about fungus. Spreading the branches on a trellis is the best way to show off this plant and increase ventilation! Get used to cutting off flowers, to have MORE flowers and better health for the plant.