My bike is getting too hard to pedal.?
Don't shoot the messenger. You bought this bike on the link? https://www.amazon.com/Murtisol-Reinforced-Commuter-Derailleur-Adjustable/dp/B07K736BV1 A) You bought one of the cheapest pieces of cr*p 💩 on the market! The frame is "HI-TEN" steel. That's the cheapest and heaviest frame material. B) JazSinc is correct in saying, "Inflate the tires. Then check to see that the brake isn't rubbing." Tires even a few pounds low on air have more ROLLING RESISTANCE making it harder. If a brake pad is rubbing the rim of a wheel, that will also make it extremely difficult. C) Who assembled the bike? You or a trained & certified bike mechanic? D) All new bikes go through a settling in process. Things get out of adjustment. Has it been TUNED-UP by a bike mechanic? If not...it's time & NOW! E) When quality folding bikes cost THOUSANDS of dollars, what did you expect for $109.99? See link... https://www.rei.com/product/133353/brompton-m6l-folding-bike?CAWELAID=120217890004754568 F) Your bike only has 6 gears. Are you using them when going uphill? When pedaling starts to become difficult - DOWNSHIFT into a lower (easier) gear. It could be at least one thing - if not many more. In short, THE INTERNET WILL NOT FIX YOUR BIKE. Take it to a REAL bicycle shop. Just Google the words "bicycle shop" with your zip code or postal code in the U.K. Estimates are FREE.
I can certainly understand frustration with garbage answers, but I count at least 5 useful responses so far. First and easiest: tire pressure, pump up your tires. Second: brake rub, easy to check, lift up each wheel off the ground and spin it by hand. If it stops quickly, your brakes may be creating too much friction. Third: gear selection, do you know how to use the gears? Bicycles require maintenance, even the most expensive ones. The less you pay for a bike the more often it will require service. If you ride any bike until things go badly wrong, they most certainly will and this will happen sooner with an inexpensive bike. Without being able to inspect your bike, nobody can do anything but guess what is wrong, and you may not be able to discern which guess is correct. Best choice? Have the bike inspected by someone who knows bike mechanics, but be warned, repairs may cost more than the price of the bike
If the link to the bike left by Hippie is correct, you bought one cheap @ss bike. It may cost more to fix it right than what you paid for it. You only get what you pay for. A basic Brompton folding bike starts around $1,300. https://us.brompton.com/bikes/brompton
Does it have a gearshift? Do you know how to use it? It sounds like it was in a low (easy) gear when you got it and now it has somehow gotten shifted into a high (harder) gear and you don't know how to shift it to the right gear for your conditions.
Do you maintain the bike properly, do you lubricate the moving parts and pump your tyres to the correct pressure, do you use the correct gears? If you are not able to maintain the bike yourself, then take it to a reputable specialist bike shop to be checked out and fixed. If you have bought a cheap bike it may simply be of such poor quality that it will not last and cannot be fixed. If you want a decent bike then Brompton is probably the number one brand, though my Dahon does just fine and costs roughly half the price of the equivalent Brompton.
Inflate the tires. Then check to see that the brake isn't rubbing.
Lots of good answers so far. Considering it's a low-end bike and you're mechanically inept, I'd say the top 3 possibilities are: 1. flat tires. Tires need to be aired up periodically. A floor pump is a necessary accessory. 2. it's in high gear. If you can't spin your pedals at 70-80 rpm without going anaerobic, you need to downshift. 3. brake rubbing rim or tire rubbing on frame. After that, all the other suggestions. Maybe the best suggestion of all was to take it to a shop if you can't see/fix the problem yourself.
Might need a new chain...
You need to grease the bearings is what I think....