I've been playing tennis for many years and have taken many lessons. I play with a friend of mine who hasn't played anywhere near the same amount nor received the same amount of instruction who I usually beat. However every time I lose a set, or a best of 3 match against him, it just makes me want to quit playing. It feels embarrassing and disappointing to lose despite having more experience than him. It makes me feel like my life has been a complete waste if someone can just pick up the same later in life and still beat me. Like what has been the point of me playing for so long, or the point of having taken lessons when someone without can still easily beat me?
Should I try to not have this mindset? If so, how. If not, should I actually just quit then?
Don't be so entitled. Tennis skill is not about years, it is about mindset and mental stability and a lot of mindful practice.
You can quit if you want to but keep in mind this happens in a lot of other sports and other places other than sports as well (e.g. like work), so if you quit tennis, this situation might not escape you and you might find yourself having similar conversations.
However, I understand where you are coming from, I have similar feelings in the past, and I was fortunate that someone helped me getting out of that mud of despair that sucks me in the further I struggle.
To escape this mud of despair, I recommend you go through the trouble to understand yourself.
A lot of times the reason you lose is not because you can't hit the shot, it is because you are fighting with your inner self, the actual self that controls the shot production.
"inner self" is a part of you that you sometimes feel it when you are making a shot that you have no idea you did it, that live in your brain that controls all the shots unconsciously, a lot of times in a match that inner self will take over everything, a lot of times it will curl up and leave your conscious mind to defend for itself. Your "inner self" also has its own personality, most of the time it is like a child, very easy to show off, be happy, but very easily bored. Learning how to tame that inner self to have discipline and knowing the time to let it go wild and crazy is important to winning in a match.
There is a reason people call Novak a mental giant, because how much he trained his inner self to be as solid and consistent as possible. Keep in mind not a lot of people do that because that is asking for self harm, but as a recreational player, all you need to do is to discover this important fact and try to approach tennis a slightly different way.
If you feel any of the following when you start losing or starting to fear to hit certain shot or keep changing tactics then I think you have similar issues that almost 100% of the tennis players would have to go through (yes, even Fed, even now!).
1. Feeling frustrated, want to curse a lot
2. Feeling weird that some shot you think normally feels like it should go in somehow doesn't go in
3. Feeling powerless, because you don't know what you should be doing at any given situation
4. Feeling tired, because you have been spending so much of your mental energy and not working thus exacerbate that situation.
I would suggest the next time you hit tennis, think and ask yourself about those questions
1. What are you really really wanting out of the game? Is it fame? Is it just beating your opponent? Is it winning a tournament? or simply just a workout?
2. How much sacrifice are you willing to make to accomplish the goal?
3. Really think about it why you feel certain emotion when you make those losing shots, a lot of those emotions are coming from your inner self btw. e.g. What is the true reason you want to hit that winner that went out? What is the real reason you are scared and want to hit a lob instead of a passing shot? etc.
4. Now if you can unravel that, ask the following questions: What is your inner self telling you when you want to hit certain shot? Are you constantly fighting against yourself on what to do with each shot? e.g. maybe you think a DTL works, but your inner self is telling you that you should hit a CC. And often times this result in a shank because your body don't know what to do.
5. What communication or key word that your inner-self can understand and execute consistently?
For me, my keyword is "control". That's the only word I need to use to start winning because in the past I often tell me inner self to "hit winner". A lot of times it is not because it is the right time for a winner but because I have very little confidence in myself to have long rally or have full control of my shot.
Someone told me this and totally change my game, I am feeling much much better and I am starting to win games and matches more consistently now. It feels really awesome.
I think you should try it.