Lemon plant infestation!

Hi all,

I've this lemon plant that I grew from seed about 5 years ago now. It was doing beautifully until very recently when it started to become infested by bugs that I can't identify...

The bugs are fat and brown stationary, usually I find them underneath the leaf or on the stem where it joins the leaf. When they're young they're a pale brown and easily squishable, but as they get older they develop a tough shell. It's almost like they're gooey things that dry out and go hard with age.

We've had this problem for about a year now, my dad and I would spend hours literally wiping these creatures off the leaves whenever I'm home (this plant is currently back at my parents house as I'm renting but I'm planning to take it whenever I buy my own place..)

I'm pretty attached to this plant, I grew it from seed just before I left for uni and now, five years later, I'd be really sad to see it go. Can anyone help me identify these critters and how to get rid of them?

Thanks!