Clunky gear changes

I have noticed my chain becoming really slack and on monday i tightened it wednesday and about 40 miles or so later it was quite slack again, im not over tightening the chain but i think i could be a bit harsh on my gear changes, i am taking my time with them now but i have noticed noisey gear changes between 1st - 3rd which i have never noticed before, normal? im also knocking it into 1st by accident alot when slowing down and not whn im at a near stand still or total standstill, oil level is a bit higher than it should be and i need to remove some but surely that wont affect it will it? (not very technicly minded yet)

Thanks

check for tight links in the chain. If you adjust it where it's tight on the loose links it'll be looser or something along those lines.

Havent noticed any tight links and i keep it well maintained, gave it a really good clean 2 weeks ago and didnt notice anything, will check it tonight, wondering if its getting to end of its life ive got no records of it ever being replaced so im guessing its same age as bike and with 19,250 miles on it

at 20,000 miles, your chain has had quite a good life I think. Look at getting a new one, with new sprockets. Mine is on its last legs now, with just over 17,000 miles on it.

Chains can last longer, but only if you like to lube every 5 minutes or can be bothered to fit a scottoiler.