If it's not going to fit, then it simply isn't going to fit. Everyone has different dimensions, and what may fit for one person may be way too big for another.
But, before having to think about returning it as useless, then try using a water based lubricant to help it along. Lie on your back with your back raised up on pillows so your bum is raised up a bit, or try using it standing up/kneeling/squatting over it. The most important thing is to relax. Just use the tip of the dildo to tease yourself with, nudging at your vagina entrance, labia and clitoris to relax yourself and work up to taking it. Use your imagination so you aren't constantly thinking about a big lump of rubbery stuff that might not fit, but rather think of it as a sex tool, think of it as a real penis, or whatever fantasy takes your fancy.
Go slowly, don't try to take it all. I think I read somewhere that the average length of a womans vagina is only 6-7cm long on the front wall and 9cm long on the rear wall. So taking length is not to be as important as a lot of people may think! Just think of taking as much as you feel comfortable with. But go slowly, putting it in just little by little and pulling out a bit before trying to go deeper. Lubricant is quite important to help it to go in. A water-based lubricant is best. Hopefully with some patience you will succeed and have a lot of fun times with your new dildo!
But if it clear that it is just too big, then you may want to think about returning it or exchanging it with something a bit smaller by contacting Lovehoney's customer service. (Links for customer service and Live Chat is at the very top of the page just above the logo.)
Good luck, and keep us updated!