It's great to see that you're making the effort to change a weakness in your game. I'll start out by saying that at 3.5, the serve is more something that will be a liability rather than a strength. In other words, you really just want to get the serve in. At 3.5, the serve isn't going to be strong enough for you to dictate a point (unlike the higher levels.....even at 5.0, most players won't dictate with their serves), and even if you put in a weak serve, your opponents are not going to be able to return efficiently enough to put you immediately on the defensive. Therefore, you might want to ditch the attempt at a big first serve and go with a 75% first serve and get a high percentage of first serves in. You say that you already get a high percentage of first serves in, but have you charted your matches? At your level, 75% first serves (or even higher) would be something good to shoot for. This means you'll need to put more spin on the ball (whether you need to take speed off your ball or not). Just remember, a spinning ball is much tougher to return than a hard flat one.
Ok, so you're a head case. ;-) Tennis is such a physically and mentally demanding sport, and it's not uncommon for mental blocks to develop. Pretty common actually. A better attitude about your serve will help, but a better attitude isn't going to help you serve with the deficiencies I see from your pics. Rather, developing a good serve will help your attitude.
That being said, there is a lot you can do to improve your serve. All the advice you've received so far has been very solid, and it seems to be working for you. From looking at your pictures, I have a few words of advice for you. And a lefty?! Wonderful! You can freak out a bunch of your opponents if you can up the efficiency of your second serve.
Ok, so it appears that you're using an Eastern grip on your serve. That is the first thing you'll need to change. You really need to switch to a continental grip to have an effective serve. You will never have a decent second serve with an Eastern grip. The grip change is the toughest thing to do, and you're going to get really frustrated with it. Get your basket and prepared to feel pretty foolish. It will certainly pay off though.
Next, the delivery on your serve needs some work. If you didn't grow up playing throwing sports, it's tough getting the hang of the service motion, which is really just a throwing motion with some minor modifications. From your pics once again, you'll need to develop that throwing motion in order to develop an effective second serve (and first serve for that matter). Here are a few drills that you can do that will really help.
Get a football and start throwing it around with a buddy. Use a football and not a baseball. A football with train your service motion to have a proper throwing motion. You can't get away "throwing like a girl" with a football, but you can with a baseball, so you really need a football for this drill. What you want to do is learn to throw a perfect spiral. Start with each of you on opposite service lines, and get the feel of tossing to each other with a highly rotating spiral. This will teach you the proper supination and pronation for the service motion, as well as the proper throwing motion, as it very difficult to throw a spiral without the proper throwing/serving motion. Once you get the hang of that, move back to opposite baselines and continue the drill. Once you get the hang of that, move even further and try throwing as far as you can. If you have a buddy that has a good arm, get him out there and have him throw with you, even if you don't play tennis together. This drill is better with someone that can throw
Next drill is a fun one, especially if you do it right after losing a frustrating match. Get an old tennis racket (one you don't mind trashing). You might want to get one from the thrift store if you don't have one laying around.
Stand at the the baseline and get ready to serve just like you always do. Toss the ball, make your service motion, and THROW the racket to the other side of the net, preferably past the opposite base line. Do this a bunch of times, so you might want to get a bunch of old rackets. This drill will teach you ACCELERATION on your serve, and they proper way to hit UP on the serve. When you translate this into a real serve, you want the same feeling of throwing the racket to the opposite side of the court. If you can get a buddy to do this with you, you can throw the racket back and forth, just be careful and pay attention. Kind of unorthodox, but these two drills will turn your serve into a weapon instead of a liability.
Just keep in mind:
None of this will do any good without the correct grip (Continental).
Higher spin rate.
Attitude and confidence with come around in time.
Lastly, make sure you're using the proper equipment. Strings now days can really help your game, especially getting more spin on your serve.
Good luck, and keep me posted on your progress.