I understand your feeling because in my almost 10 years of DotA I've been there aswell. As you state there is no best response to your problem than practicing. In DotA you never stop learning and even though you feel you are no longer getting better, you are still getting more and more experienced with every game and in the long run you will be able to overcome this problem, given that you want to get better of course.
Now to answer your problem direclty there are several things you can do.
1) Read guides
You've been playing for over a year which means you have the basics of the game's mechanics. You can now start reading complete and full guides of heroes that you have trouble understanding. Every hero in the game has one or several guide on the official dota site. Those guides often describe how to play the hero and how to behave in the different common situation that you'll be facing in a DotA game. The guides also list their strength and weakness and how to overcome them. Some might be outdated but that doesn't mean the advice you'll find in them are outdated. Start reading !
2) Learn to recognise a threat
Playing a squishy hero is hard because you often become an easy target for hero with lots of burst damage. If you start playing too cautiously you'll fall behind in XP and gold and fail anyway. The first thing you have to do when playing a squishy hero is to recognise on the enemy team which hero is a threat why and when.
By when I mean when in the game will this hero try to kill me ?
The enemy mid is not likely to come early in your lane however the support can rotate and try to gank you.
By why I mean what ability/kind of damage makes this hero a threat
If I can go invisible, can he detect me ? Does he deal magical/physical damage ? Is he a good chaser ? Does he have a lot of disable ? etc.
Once you've answered those questions you can play accordingly and aim for a defensive item suited for the kind of threat you are facing.
This lead to...
3) Itemization and skill built
You have several ways to "toughen" you up in the early game. Here's a list of common items you can get to help you survive and sustain if you are being harassed :
- Magic Wand : the most cost efficient item in the game. This can and will save your life countless time. Buy it. You can bait enemy into coming closer to the tower to finish the kill when in fact you still have plenty of HP to restore. You can make the enemy stop chasing you if they think this last attack will kill you when in fact at the last moment you regain some needed HP. You can use it to restore your mana and use another spell. Just buy it.
- Stout shield / Poor's Man Shield : helps a lot versus physical damage
- Tranquil Boot : +4 armor helps versus physical damage. The movespeed and regen is nice and can help you sustain a lot.
- Bracers : you can buy one and upgrade it later into drums of endurance. This bring some much needed HP
Another thing that is often overlooked is that tankiness is not the only way to go when you are squishy. Mobility is the key :
- A Blink Dagger can help you reposition/escape very easily. Same goes with Force Staff. I always get one of those even if I'm playing a carry Sniper because the positionning is just so important.
- Eul's Scepter gives you some movespeed aswell as a disable versus potential chasers.
- I would recommand against Shadow Blade since this item is easily countered by dust and sentry wards that cost respectively 180 and 200 gold (affordable for an ennemy support) and render this 3000 gold item utterly useless as an escape.
When picking your skill take a few second to decide if you want to :
- Max your escape if they have good chaser
- Max a defensive spell
- Level stats if you need more HP and one of your spell is of no use this early in the game
- Max your disable
Those decision impact the way you can play and let you be more agressive if done right.
4) Map awareness and positionning
Last but not least : knowing that you are safe or not is important. Having wards placed at strategic path that enemy will most likely take if they try to gank/kill you is vital. If you know where they are, you know how you can play. If you can't see them : play safe. However if you know where they are you can either keep on farming, go get a rune without risk, in other word play as you wish since you know you are not in danger.
When it comes to teamfight the key with squishy heroes is positionning. You are squishy, you are a target => don't put yourself in danger. By that I mean don't go first, you have (or at least should have) some kind of initiation in your team. Let the initiator (Tidehunter, Earth Shaker, Void, SandKing, etc.) do his job. Let the heroes in your team that control the teamfight (Brewmaster, Clockwerk, Doom, Axe, etc.) do their jobs and position yourself accordingly. They are supposed to be the target so let them be, stay behind and when your hero comes into play (either by using your skill if you are a Lina or by atacking if you are a sniper) you can follow in the melee and start doing work. This is why Blink Dagger/Force Staff are so important : they allow you to jump in and out of fight and reposition quickly when needed.
In a nutshell
If you want to get better, you better start reading. Practice makes perfect but theorical knowledge helps a lot. Playing DotA is all about decision making, force yourself to analyse the game and ask yourself the question about how is the game developping and how you should play according to that. Finally if someone is bullying you => mute them. Let them say one bad thing to you and let that be it. You can't be reported for being bad however you can report them for communication abuse. Being bullied and letting someone take your moral down is an assured defeat so try to stay in a positive state of mind and mute toxic players.
