Is there a limit on how often you should use them? I always notice when I first use a new oil it works at first and then wears off after a while. Would using them too much like everyday make the magick backfire?
No, but over time oils loose their energy because it's old. Think of it like preserves, it lasts longer, but after a while the 'freshness' wears off. [Dried herbs and oils to me have a 1 year shelf life]
Casting a spell on a daily basis might be a little much, but you can do it provided you have enough energy to cast. It sounds to me however you're overdoing it on the spells. Sometimes spells don't work when you ask too much, casting for the same thing every day is like asking your parents for a puppy every day, they're going to get annoyed and start to ignore you. Once is typically enough.
How are you storing the oils? How an oil is stored can be detrimental to its shelf life and the level of its effectiveness. When storing oils, make sure to keep them in a cool, dry, dark environment. If you leave oils in a constantly changing environment (such as taking an oil around in a purse) or overly warm environment, they will lose their efficacy extremely fast. Should you carry the oils around with you every day, expect to replace them every few weeks.
Is your altar exposed to light to prolonged periods of time? Sunlight can weaken the effectiveness of oils at a pretty remarkable rate, although mad made light can have the same effect at a similar rate (depending on the level of lighting, and the type of light source [i.e. firelight, lamp light, led light, etc.] )