Sparring is absolutely essential to learning how to deal with live energy. I would like to caution you though. Sometimes sparring can create bad habits what I mean by this is:
To often a practitioners go into sparring with the wrong mindset. They simply try and unload on their opponent.
If you are doing this without protection gear someone is going to get hurt pretty quickly and if you are doing this with gear on, then you aren't realistically experiencing the reality of fighting.
The example I would like to give you is put head gear on and a mouth piece. Then have your training partner put gloves on, then have him punch you as hard as he can.
In that example you will be able to take the shot for the most part. Obviously a shot to the nose can still break your nose. The point that I am trying to make is that it's not realistic.
Sparring is a tool to help prepare you for a tournament, it teaches you how to respond to a violent situation and develops your timing.
Now back to your current issue:
The best time to attack someone is when they are attacking you. Because in order to attack you they have to close the distance.
It sounds like you are reaching in to try and get a strike, when someone does that it's easy to read and it doesn't take to much to shuffle step out of range or cut an angle out of range.
The #1 thing sparring teaches you is timing. You have to develop the ability to time your strikes whilst your opponent is invading your space.
If you chase the person they are the ones choosing where they want to fight. Think about that for a moment. There is a difference between chasing your opponent and making a calculated attack. That's for another discussion though
Finally, unfortunately there are people out that don't respect technique and will keep coming and coming.
The only way this type of person will stop coming is if you hurt them enough to slow them down. Unfortunately I've had this happen to me, eventually I gave the guy a black eye. Not out of anger but he really gave me no other choice. After that happened, the sparring sessions got lighter.
I don't recommend that approach though because it can create bad blood. I recommend trying to resolve any problems before they even occur.
Before you start sparring with someone you should talk with the person about what is acceptable to you and what is not. Your martial arts training is about YOU so you decide how much you want to take.
Your partner should respect that. There is nothing wrong with extreme training however it's not for everyone and the truth is it's not needed to learn how to defend yourself should the situation comes up.
The most important thing to consider here is communication. Both you and your partner should know the objective of the sparring session.
Some of my sparring sessions where battles of endurance, some of them where for working on timing and sometimes they where just to see how much punishment I could take... In all cases it's important to understand what the goal is of the training session.
One final bad habit to avoid. It's important that you don't create habits like leaning forward or throwing sloppy punches to just try and get a strike.
If there is one thing that is true, you will fight how you spar. So if you aren't constantly working on having good technique, when you fight your technique will be poor.
Hope this answer helps some...