First off you need to understand why stun locking occurs. It's really stupid and directly tied to your latency. Say you have 100ms latency. When you attack, you aren't actually attacking until 100 milliseconds later because that's how long it takes for the information to go from the client (your PC) to the server. Let's pick an arbitrary point in time and call it T. You attack at T+50ms. The server receives it at T+150ms. However, another player attacked you at T+100ms server time. You don't see this until T+200ms, which is way after you attacked in your view, but the server thinks you were the second person to attack. As a result, the server cancels your attack because you were hit between the time you pressed the attack button and when the server received this information.

Why cancelling actually occurs like this is a design decision by Three Rings and it's not one I can get behind for PvP but I can imagine they found it to be the more reasonable choice for PvE. It would also be far less noticeable in PvE where enemies are far more predictable.

Now, as to how to avoid being stun locked:

• Don't rely on your shield all the time until it breaks. Being hit on the shield causes a small amount of hitstun, which gives you less time to counter and move.
• Predict your opponents. If they're for example going around in a circle around you, it's very easy to tell where they'll try to hit you next. To counter this, you can try to move into a position where they won't quite hit you from their next position, and then you slap them in the face when they miss.
• Guardian and Recon are forced to play defensively against strikers because the Striker is the one who chooses where and when fights happen. However, when you throw in guns and bombs, you get a lot of options for denying area, which can let you turn the fight in your favor by e.g. holding the camera advantage, denying them the point, or stalling until you can get help from a teammate.
• Avoid fighting competent players alone. If you know you can't beat them alone, there's usually no good reason to approach them without backup, it'll just result in you dying. Also remember that Strikers are punished a lot less by death and have an easy time going back for healing since they're so fast.
• Mobility is the greatest boon a guardian can have. Play with Black Kat gear and Warmaster Rocket Hammer if it fits the situation.